(Help) Cross domain tracking leads / registrations

Current lead flow is like this:

Lead lands on domain 1 landing page, clicks a button to open a whatapp chat ( because hes interested) -> lead receives further information and link to domain 2 website where he fills out a form -> success page on domain 2.

Now, I have already a google tag manager account and domain 1 GTM is installed:
Currently im tracking only the button click on domain 1 that opens up the whatsapp chat. With this I have been getting leads, but also alot of false data to my campaigne, as not everybody that clicks the button actually writes a message to the whatsapp, hence my traffic went kind of to trash / very inconsisten.

My question is, how do I set up cross domain tracking that the final conversion action that counts is the success page visit on domain 2 in connection that the lead firstly clicked that button on domain 1, when my running campaigne is domain 1 landing page? and is this even allowed to track conversions on a domain that is not activly advertised?

and / or does it break the converion flow, that they receive a link manuelly from my in the whatsapp chat?

happy to hear from the experts on here, cheers!

Nobody an idea?

vitamino said:

Nobody an idea?

Click to expand...

Hello "vitamino" my name is Ryan, I'm from Brazil, a country in South America that speaks Portuguese, use a direct redirect, which sends native people to WhatsApp instead of the lead having to click on several buttons to send you directly.