Hi there @hami009,
While we cannot provide full details on how to integrate with other form plugins, there are several that have built-in integrations with MailPoet to make your life easier. You can find a list of these here:
https://kb.mailpoet.com/article/198-forms-themes-plugins-that-work-with-mailpoet
If you prefer to keep using WP Job Opening Plugin, I’d recommend reaching out to the developers of that plugin for assistance. If there is a way to connect the two forms, they should be able to walk you through it.
As a hint, I’d recommend using MailPoet itself to create a form that connects to the list you’d like, and embedding it on a test page. Then, view the source of that page and it should show you the required MailPoet form fields, such as the form action, email address input string, and hidden fields (of which there will be several). These are the values you’d use to connect with a third party form.
As I mentioned, however, we would not be able to fully walk you through the process or debug any connection issues. Please contact your third party form developer for further assistance.