In magento 2 how to add custom options in action apply field in cart price rule form in admin

This Dropdown options render from getMetadataValues() function which is available in Magento\SalesRule\Model\Rule\Metadata\ValueProvider.php

So, You can create plugin for that because, it's public method. I created plugin for that and get output as like you want.

Follow this below steps for that :

1) Create di.xml to set plugin at app/code/RH/CustomPlugin/etc/adminhtml :

<?xml version="1.0"?>
<!--
/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */
-->
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <type name="Magento\SalesRule\Model\Rule\Metadata\ValueProvider">
      <plugin name="salesrule-plugin" type="RH\CustomPlugin\Plugin\Rule\Metadata\ValueProvider" sortOrder="1" />
    </type>
</config>

2) Now, create plugin file ValueProvider.php for add custom option for dropdown at RH\CustomPlugin\Plugin\Rule\Metadata:

<?php

namespace RH\CustomPlugin\Plugin\Rule\Metadata;

class ValueProvider {
    public function afterGetMetadataValues(
        \Magento\SalesRule\Model\Rule\Metadata\ValueProvider $subject,
        $result
    ) {
        $applyOptions = [
            'label' => __('Popular'),
            'value' => [
                [
                    'label' => 'The Cheapest, also for Buy 1 get 1 free',
                    'value' => 'buy-1-get-1-free',
                ],
                [
                    'label' => 'Get $Y for each $X spent',
                    'value' => 'get-y-for-each-x-spent',
                ],
            ],
        ];
        array_push($result['actions']['children']['simple_action']['arguments']['data']['config']['options'], $applyOptions);
        return $result;
    }
}

Output :

You can modify option in array as you want.

Hope, it will helpful for you.