Add Return URL and Cancel URL Options about divi-paypal

If you need to see a sample of this we've done something similar in our PayPal for WooCommerce plugin. You can set the Cancel URL the way I'm requesting here.

@keyurptl I see you added the Return URL and Cancel URL fields here, but you just did it as text fields where the user would have to find the URL and copy/paste it into the field.

As mentioned above, what I want here is the ability for people to select from a drop down list of all the pages they have created in WordPress. They should see all the page titles in the drop down, and they could just choose that way, but the full URL would be used as the value for the HTML form.