How to set target on iframe form (form from other site)?

Question

JavaScript

We are at a dead end, we are looking for a solution to our problem, I will be glad if you can tell me.
The task is to set up the tracking goal on the booking form on the utesov-hotel.ru website (the form is in the header, this is not our website, but everything is the same). The booking module itself (and the form) is on another site anapais.ru and works through an iframe.
How to set up a "JavaScript event" target on a button or form fill? or "Visit pages"... It doesn't matter at all, the main thing is to track the sources of successful transactions...

As far as I understand, in the form you can only put a unique html text on the confirmation page - ask the developers of this form. Maybe there is something so tricky to come up with, maybe a hidden opening of some page on our site can be done? Or hang a hidden button ...
The developers of the form have several clients and I think we can agree to come up with something for the common good, so to speak.

How can we solve this, where to dig?

Answer 1

As a result, the developers were able to hang onclick on the button and added the counter to themselves. Now we can see all conversions in the iframe.

Answer 2

Pass some GET parameter with a feedback method. For example URL.
Something like this:
You can add more options. For example, indicate what you want to be sent to you.
Form developers send you what you need in any way they want. For example, they pull the URL from your site and pass GET parameters:
https://mysite.com/fromform.php?answer=123