Is there a way to disable the submit button/ grey out the parent screen on opening a URL in a window until that window is closed? Or is there an approach to launch an external website in another window/ pop up and grey out the parent window/ Submit button? Modal dialog did not work as I could not open the form on the external website and submit it.
Looking at the OOTB options for the auto-generated controls, I don't see anything that jumps out at me (which I'm sure is why you're asking ).
One thing I can think of to get you halfway there is adding an action to set a parameter that's evaluated by a disable when condition to disable it on the click that launches the window. Then it would be a matter of figuring out how to re-enable when you close the window for the external site.
If you use a parameter, a simple refresh section is all you would need to wipe out the parameter and enable the button.