Magento – How to add new input field in cart page in magento2

Magento provides additional attribute functionality to pass additional data. On cart page, create input field for remark then in custom module's events.xml, create an observer like below :

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd">
    <event name="checkout_cart_product_add_after">
        <observer name="customremark" instance="Companyname\Modulename\Observer\customremark" />
    </event>

</config>

Then in observer, you can get this remark field from request

<?php

namespace Companyname\Modulename\Observer;

use Magento\Framework\Event\ObserverInterface;
use Magento\Framework\App\RequestInterface;

class SetAdditionalOptions implements ObserverInterface
{
    /**
     * @var RequestInterface
     */
    protected $_request;

    /**
     * @param RequestInterface $request
     */
    public function __construct(
        RequestInterface $request
    ) {
        $this->_request = $request;
    }

    /**
     * @param \Magento\Framework\Event\Observer $observer
     */
    public function execute(\Magento\Framework\Event\Observer $observer)
    {
        // Check and set information according to your need
        if ($this->_request->getFullActionName() == 'checkout_cart_add') { //checking when product is adding to cart
            $product = $observer->getProduct();
            $additionalOptions = [];
            $additionalOptions[] = array(
                'label' => "Remark",
                'value' => "Your Information",
            );
            $observer->getProduct()->addCustomOption('additional_options', serialize($additionalOptions));
        }
    }
}

Hope this helps.

For Custom attribute you needto use layoutprocessor and custom extension attribute.

Here is the good example, you can use it.

https://github.com/dckapblog/Custom-field-in-checkout-page