Form conversion on another website


we have a website that belongs to us on which we have added an HTML contact form hosted on another site that does not belong to us. on our site we have matomo and i would like to do a conversion when a person clicks on submitted form. But how to implement this knowing that the form that appears on our site is not hosted with us.

Sorry I don’t understand the question…

I just edited my post hoping it would be clearer.

How is this implemented? With iframe? And then, after submitting the form, what happens for the user (redirected to an internal page? to an external page? nothing because the form is sent via ajax to the third party? etc.?)

yes it is an iframe in wordpress. No forwarding to another page, just a message: Your request has been sent. You will be contacted very soon.

The problem is: If you host the tracking code in some page, then you can’t track anything located in the page <iframe>, except if you add also the tracking code also in the <iframe> content… (this is done by browsers for security purposes).
What happens after the form has been submitted? Probably nothing else than a submission message…?