Create Custom Category Dropdown in Admin – Magento2 Guide

create a module Yournamespace_CategoryAttribute.

in app/etc/Yournamespace_CategoryAttribute.xml

<config>
  <modules>
    <Yournamespace_CategoryAttribute>
      <active>true</active>
      <codePool>local</codePool>
    </Yournamespace_CategoryAttribute>
 </modules>
</config>

===========================================

now if you want to use form, in the same module:

in config.xml

<?xml version="1.0"?>
<config>
    <modules>
        <Yournamespace_CategoryAttribute>
            <version>0.1.1</version>
        </Yournamespace_CategoryAttribute>
    </modules>
    <global>
        <models>
            <categoryattribute>
                 <classYournamespace_CategoryAttribute_Model</class>
            </categoryattribute>
            <!-- removed eav_entity_setup rewrite -->
        </models>
        <blocks>
            <categoryattribute>
                <class>Yournamespace_CategoryAttribute_Block</class>
            </categoryattribute>
        </blocks>
    </global>
    <frontend>
        <routers>
            <categoryattribute>
                <use>standard</use>
                <args>
                    <module>Yournamespace_CategoryAttribute</module>
                    <frontName>categoryattribute</frontName>
                </args>
            </categoryattribute>
        </routers>
        <layout>
            <updates>
                <yournamespace_categoryattribute>
                    <file>yournamespace_categoryattribute.xml</file>
                </yournamespace_categoryattribute>
            </updates>
        </layout>
    </frontend>
</config>

in YourNamespace/CategoryAttribute/Model/Setup.php

<?php
class Yournamespace_CategoryAttribute_Model_Setup{

    public function setAdminAttribute($params){
        $setup = new Mage_Eav_Model_Entity_Setup('core_setup');
        $setup->startSetup();

        $setup->addAttribute('catalog_category', $params['attr_code'], array(
            'group'         => $params['section'],
            'input'         => $params['input_type'],
            'type'          => $params['db_type'],
            'label'         => $params['attr_label'],
            'backend'       => '',
            'visible'       => 1,
            'required'      => 0,
            'user_defined' => 1,
            'global'        => Mage_Catalog_Model_Resource_Eav_Attribute::SCOPE_GLOBAL,
        ));

        $setup->endSetup();
    }

}

in your controllers/IndexController.php

<?php
class Yournamespace_CategoryAttribute_IndexController extends Mage_Core_Controller_Front_Action{

    public function indexAction(){
         $this->loadLayout();             
        $this->renderLayout(); 
    }
    public function createAction(){
        $formParams = $this->getRequest()->getParams();
        Mage::getModel('categoryattribute/setup')->setAdminAttribute($formParams);
    }

}

in Yournamspace/CategoryAttribute/Block/CategoryAttribute.php

<?php
class Yournamespace_CategoryAttribute_Block_CategoryAttribute extends Mage_Core_Block_Template
{

}

in app/design/frontend/your_theme/default/layout/yournamespace_categoryattribute.xml

<?xml version="1.0"?>
<layout version="0.1.0">
    <categoryattribute_index_index>
        <reference name="content">
            <block type="categoryattribute/categoryattribute" name="categoryattribute" template="categoryattribute/categoryattribute.phtml" />
        </reference>
    </categoryattribute_index_index>
</layout>

in app/design/frontend/your_theme/default/template/categoryattribute/categoryattribute.phtml

put your form and the form-action should be $baseurl/categoryattribute/

Note: This can be organised from Admin panel, by adding a admin grid and post these from admin form.

Inside your form container add button (in the _construct method):

class Edit extends \Magento\Backend\Block\Widget\Form\Container
{
//....
    /**
     * Initialize form
     * Add standard buttons
     * Add "Save and Continue" button
     *
     * @return void
     */
    protected function _construct()
    {
    //.....


        // Here we check is that model already exists
        if ($this->getRequest()->getParam('id')) {
            // creating duplication url
            $duplicateUrl = $this->_urlBuilder->getUrl(
                static::URL_PATH_DUPLICATE, // path to the duplicate action, string like 'module_route/controller/action'
                [
                    // our model's id, id - is parameter from the request
                    'id' => $this->getRequest()->getParam('id'),
                ]
            );
            $this->buttonList->add(
                'duplicate',
                [
                    'class' => 'save',
                    'label' => __('Duplicate'),
                    'onclick' => 'setLocation("' . $duplicateUrl . '")'
                ],
                12 // sort order
            );
        }

Add controller's action:

namespace Vendor\Module\Controller\Adminhtml\Some\Thing;

class Duplicate extends \Magento\Backend\App\Action
{
    /**
     * Create model duplicate
     *
     * @return \Magento\Backend\Model\View\Result\Redirect
     */
    public function execute()
    {
        /** @var \Magento\Backend\Model\View\Result\Redirect $resultRedirect */
        $resultRedirect = $this->resultRedirectFactory->create();
        /** @var \Vendor\Module\Model\Name $model */
        $model = $this->coreRegistry->registry('your_model_unique_identifier');
        try {
            $newModel = clone $model; // clone
            $newModel->setId(null); // remove id
            $newModel->isObjectNew(true); // set object as new
            $newModel->setData('is_active', 0); // set another data if needed
            $newModel->getResource()->save($newModel); // save new model (clone)
            // add success message and redirect to the edit form with the newly created model
            $this->messageManager->addSuccessMessage(__('You duplicated the model.'));
            $resultRedirect->setPath('*/*/edit', ['_current' => true, 'id' => $newModel->getId()]);
        } catch (\Exception $e) {
            // if something is going wrong: catch exception & redirect back
            $this->_objectManager->get('Psr\Log\LoggerInterface')->critical($e);
            $this->messageManager->addErrorMessage($e->getMessage());
            $resultRedirect->setPath('*/*/edit', ['_current' => true]);
        }

        return $resultRedirect;
    }
}

Result should look like this: