Magento – How to call observer before checkout page in magento2

define(
[
    'jquery',
    'Magento_Ui/js/modal/modal'
],
function($) {
    "use strict";

    $.widget('mage.modalForm', {
        options: {
            modalForm: '#modal-lens-form',
            modalButton: '.open-modal-form'
        },
        _create: function() {
            this.options.modalOption = this._getModalOptions();
            this._bind();
            this._hide();
        },
        _getModalOptions: function() {
            /**
             * Modal options
             */
            var options = {
                type: 'popup',
                responsive: true,
                title: 'Select Product'
            };

            return options;
        },
        _bind: function(){
            var modalOption = this.options.modalOption;
            var modalForm = this.options.modalForm;

            $(document).on('click', this.options.modalButton,  function(){

                if ($('#product_addtocart_form').valid()) { //Open modal only if validation passes on base product form
                    //Initialize modal
                    $(modalForm).modal(modalOption);
                    //open modal
                    $(modalForm).trigger('openModal');
                }    
            });
        },
        _hide: function(){
            $(this.options.modalForm).hide();
        }
    }



    );

    return $.mage.modalForm;
}
);

Form in form.phtml

    <div id="myModel">
        <form action="<?php  echo $block->getBaseUrl().'your path to controller';?>" method="post" id="form_validate contact-form" data-mage-init='{"validation": {}}'>
            <label>Name</label>                
            <input type="text" name="name" id="name" data-validate="{required:true}">
            <label>Quantity </label>
            <input type="number" name="quote_qty" id="quote_qty" data-validate='{"required":true}'>    
            <label>price</label>
            <input type="text" name="quote_price" id="quote_price"  data-validate="{required:true}">
            <label>description</label>
            <input type="text" name="quote_desc" id="quote_desc" data-validate='{"required":true}'>
            <input type="hiddden" name="product_price" id="product_price" hidden>                 
            <button type="submit" id="submit" name="submit">submit</button>
            <button type="reset" name="reset">Reset</button>
        </form>
    </div>   
    <script type="text/x-magento-init">
        {
            "#custom-form": {
                "validation": {}
            }
        }
    </script>

Call this form.phtml into model.phtml

<button  class="open-contact-form">Quote This Product</button>
<div id="popup-modal">
    <div class="contact-form-popup" style="display: none;">
        <?php           
            echo $this->getLayout()
            ->createBlock("Infy\Quotesystem\Block\Quote\Quote" )
            ->setTemplate("Infy_Quotesystem::form.phtml")
            ->toHtml();             
        ?>
    </div>
    <div class="static-block-message" style="display: none;">
    <?php echo 'display error'; ?>
    </div>
</div>
<script>
    require(
    [
    'jquery',
    'Magento_Ui/js/modal/modal'
    ],
        function($, modal) {
            jQuery(document).ready(function(){
                var options = {
                    type: 'popup',
                    responsive: true,
                    innerScroll: true,
                    title: 'Contact Form',
                    buttons: [{
                    text: $.mage.__('Close'),
                    class: '',
                    click: function () {
                    this.closeModal();
                    }
                     }]
                };
                var openModal = modal(options, $('#popup-modal'));

                jQuery('body').on('click', '.open-contact-form', function(){
                    jQuery('.contact-form-popup').show();
                    jQuery('.static-block-message').hide();
                    jQuery('#popup-modal').modal('openModal');
                });

                jQuery('body').on('click', '#contact-form .action.submit', function(e){
                    e.preventDefault();
                    e.stopImmediatePropagation();
                    jQuery.ajax({
                    type: 'post',
                    url: '<?php  echo $block->getBaseUrl().'Your path to controller';?>',
                    data: jQuery('#contact-form').serialize(),
                    cache: false,
                    showLoader: 'true',
                    success: function(data, status, xhr) {
                            // data contains your controller response
                        },
                    error: function (xhr, status, errorThrown) {
                        console.log('Error happens. Try again.');
                        console.log(errorThrown);
                    }
                    });
                    return false;
                });
            });
        }
);
</script>

Create your controller

namespace Vender\Modulename\Controller\Index;

use Magento\Framework\App\Action\Context;
use Magento\Framework\Controller\ResultFactory; 

class Controllername extends \Magento\Framework\App\Action\Action
{


    protected $_pageFactory;

    public function __construct(        
        \Magento\Framework\App\Action\Context $context, 
         \Magento\Framework\Controller\ResultFactory $resultFactory,
        \Magento\Framework\View\Result\PageFactory $pageFactory,
         \Infy\Quotesystem\Helper\Data $helperData,
         \Infy\Quotesystem\Model\PostFactory $postFactory
    )
    {
        parent::__construct($context);
        $this->_pageFactory = $pageFactory;
        $this->resultFactory = $resultFactory;
        $this->_postFactory = $postFactory;
        $this->helperData = $helperData;

    }

    public function execute()
    {
        $resultPage = $this->_pageFactory->create();
        $post = $this->getRequest()->getPostValue();    
        $qty = $this->getRequest()->getParam('quote_qty');        
        $price = $this->getRequest()->getParam('quote_price'); 
        $desc = $this->getRequest()->getParam('quote_desc');
        $p_name = $this->getRequest()->getParam('name');
        $p_price = $this->getRequest()->getParam('p_price');

        $objectManager = \Magento\Framework\App\ObjectManager::getInstance();
        $customerSession = $objectManager->create('Magento\Customer\Model\Session');
        if ($customerSession->isLoggedIn()) {            
            $emailcust = $customerSession->getCustomer()->getEmail();            

            $std = $this->_postFactory->create();
            $std->setProduct_quantity($qty);
            $std->setQuotePrice($price);
            $std->setQuoteDescription($desc);
            $std->setProductName($p_name);
            $std->setProduct_price($p_price);
            $std->setCustom_email($emailcust);

            $adminuserId = 1;
            $user1 = \Magento\Framework\App\ObjectManager::getInstance()->create(
            'Magento\User\Model\User');
            $user1->load($adminuserId); 
            $name = $user1->getUsername();
            $email = $user1->getEmail();
            $msg = 'User Add Quote in '.$p_name.' Quote Price is: '.$price;
            $user = $emailcust;
            if ($std->save())
            {
                $sendmail = $this->helperData->sendMail($msg, $user, $p_name, $name, $email);
            }
            else{$this->messageManager->addError(__($e->getMessage()));}


            // $resultJson = $this->resultFactory->create(ResultFactory::TYPE_JSON);
            $resultRedirect = $this->resultFactory->create(\Magento\Framework\Controller\ResultFactory::TYPE_REDIRECT);
             $resultRedirect->setPath('*/index/*');
            return $resultRedirect;


        }
        else{
          $response = $this->resultFactory
            ->create(\Magento\Framework\Controller\ResultFactory::TYPE_JSON)
            ->setData([
                'status'  => "Incomplete",
                'message' => "Please Login/Register!!"
            ]);

        return $response;

        }   
    }
}