This happens because you are trying to open a window without user interaction, you send a form - this is interaction - but after you receive a response to the request, you try to do this without direct user interaction. As a result, the blocker is triggered

You can make a modal window that will open when you click on the form submission and spin the loader. After you receive a response - you will show a button when clicked on which you will call window.open(...) or even add a link with the attribute target='_blank'