How do I make an iframe form submission trigger a "thank you" page on the main page?

See more:

HTML

forms

IFrame

trigger

tracking

I need to code an iframe on my site to trigger a "thank you" page on the main page where it is embedded because I don't have access to the source code of the form that is iframed.

What I have tried:

I saw a forum here asking the same question, and the only solution listed was this line of code:

top.location = '<your redirect url>';


However, I have no idea where that would even go in my iframe code which is:

<iframe loading="lazy" src="https://cancare.volunteerportal.org/?nd=user_volunteer_application&entry_point_type=global_entry_point&entry_point_id=2" width="950" height="2000"><br />
</iframe>

If someone could let me know how to code this or if you have a better way of doing this PLEASE let me know. This 3rd party form hosting site is making my life very hard.

Thank you in advance!