add-on.php

<?php

/**
 * Plugin Name: Test Add-on
 */
add_action('wp_enqueue_scripts', function () {

	wp_enqueue_script(
		'cf-add-on-test',
		esc_url_raw(plugins_url('test-add-on.js', __FILE__)),
		[
			//'wp-blocks', 'wp-element', 'wp-editor'
			'jquery',
		],
		md5_file(__DIR__ . '/' . 'test-add-on.js'),
		true
	);
});

add_filter('-caldera_forms_render_get_form', function ($form) {
	if ('CF5d4835db87edb' !== $form['ID']) {
		return $form;
	}

	$id = 'stripe_token_' . $form['ID'];

	$form['fields'][$id] = [
		'ID' => $id,
		'label' => 'stripe-token',
		'type' => 'hidden',
		'slug' => $id,
		'conditions' => ['type' => '',],
		'caption' => '',
		'config' =>
		[
			'custom_class' => '', 'placeholder' => '', 'default' => '',
		]
	];

	$form['layout_grid']['fields'][$id] = '1:1';


	return $form;
});


/**
 * On form submit, do a redirect to a diffrent site, then complete submission
 */
add_action('caldera_forms_submit_process_start',  function (
	$form,
	$referrer,
	$process_id
) {
	//If we have a already done redirect, do nothing
	if (isset($_GET['cf_tp'])) {
		return;
	}

	//Get current URL
	$url = caldera_forms_build_url($referrer);
	//Append transient ID
	$url = add_query_arg('cf_tp', $process_id, $url);

	//Set CF to redirect to other site
	add_filter('caldera_forms_redirect_url_complete', function () use ($url, $process_id) {
		//Save transdata
		//This should be automatic. See: https://github.com/CalderaWP/Caldera-Forms/issues/3312
		global $transdata;
		Caldera_Forms_Transient::set_transient($process_id, $transdata, 5 * 60);

		//create redirect URL.
		return 'http://localhost:8080/?redirect=' . urlencode($url);
	}, 9999);
}, 10, 3);

//@see https://github.com/CalderaWP/Caldera-Forms/issues/3313
if (!function_exists('caldera_forms_build_url')) {
	function caldera_forms_build_url(array $parts)
	{
		if (function_exists('http_build_url')) {
			return http_build_url($parts);
		}
		return add_query_arg(
			$parts['query'],
			(isset($parts['scheme']) ? "{$parts['scheme']}:" : '')
				. ((isset($parts['user']) || isset($parts['host'])) ? '//' : '')
				. (isset($parts['user']) ? "{$parts['user']}" : '')
				. (isset($parts['pass']) ? ":{$parts['pass']}" : '')
				. (isset($parts['user']) ? '@' : '')
				. (isset($parts['host']) ? "{$parts['host']}" : '')
				. (isset($parts['port']) ? ":{$parts['port']}" : '')
				. (isset($parts['path']) ? "{$parts['path']}" : '')
				. (isset($parts['fragment']) ? "#{$parts['fragment']}" : '')
		);
	}
}

docker-compose.yml

version: '3.1'

services:

  wordpress:
    image: wordpress
    ports:
      - 8228:80
    environment:
      WORDPRESS_DB_PASSWORD: example
      WORDPRESS_DEBUG: 1
      ABSPATH: /usr/src/wordpress/
    volumes:
      - wordpress:/var/www/html
      - .:/var/www/html/wp-content/plugins/caldera-forms
      - ./wp-content/plugins/add-on:/var/www/html/wp-content/plugins/add-on
  cli:
    image: wordpress:cli
    volumes:
    - wordpress:/var/www/html
    - .:/var/www/html/wp-content/plugins/caldera-forms
    - ./wp-content/plugins/add-on:/var/www/html/wp-content/plugins/add-on
    environment:
      WORDPRESS_DB_PASSWORD: example
      ABSPATH: /usr/src/wordpress/
  mysql:
    image: mysql:5.7
    environment:
      MYSQL_ROOT_PASSWORD: example
      MYSQL_DATABASE: wordpress_test

  wordpress_phpunit:
    image: chriszarate/wordpress-phpunit
    environment:
      PHPUNIT_DB_HOST: mysql
    volumes:
      - .:/app
      - testsuite:/tmp

  composer:
    image: composer
    volumes:
      - .:/app

volumes:
  testsuite:
  wordpress:

test-add-on.js

jQuery(document).ready(function($) {
  //variable to prevent blocking all
  var blockedOnce = false;
  $(document).on("cf.ajax.request", function(e, obj) {
    //only block submission once
    if (!blockedOnce) {
      //Make sure window.cf2 is set properly
      if (!window.cf2) {
        window.cf2 = {};
      }
      var formId = obj.formIdAttr;

      if (!window.cf2[formId]) {
        window.cf2[formId] = {};
      }

      if (!window.cf2[formId].hasOwnProperty("fieldsBlocking")) {
        window.cf2[formId].fieldsBlocking = obj.fieldsBlocking || [];
      }
      //end hacks caused by not paying down this debt before CF Stripe update

      //block submission based on a field
      window.cf2[formId].fieldsBlocking.push("fld_6866745_1");
      //Set flag to prevent recurssion
      blockedOnce = true;

      //For testing purposes, re-submit in 2 seconds
      setTimeout(function() {
        //unblock
        window.cf2[formId].fieldsBlocking = [];
        //resubmit
        obj.$form.submit();
      }, 2000);
    }
  });
});


var checkedout = false;

//submit form
//redirect back to same url with cf_tp query arg
//this triggers submit again, redirect to Stripe
//redirect back to client, triggers submit again
$( ).on( 'submit-or-whatever', () =>{
    if( 'cf_tp query args' ){
            if( 'has Stripe tokem' ){
                //Step 3 -> have saved transient, have stripe token.
                //submit normal
            }else{
                //step 2 -> have transient, not the stripe token
                //redirect to stripe chekout using current url (with cf_tp query arg) as success_url for stripe.redirectToCheckout()
            }
    }else{
        //submit form -> step 1 -> no transient
    }
});