I have two domains: mainsite.example.com and signup.example.com.
The former is built by marketing, has SEO, and keyword tracking using Google Ads. The latter has a Google Tag Manager integration. When people discover example.com, they land on mainsite.
Assuming there is only one possible referrer from the mainsite leading to the signup page. If someone signs up on signup.example.com, how do I know the keywords or ads that led to a signup conversion across these two domains?
Right now I count the signups in Google Analytics and linked the data Google Ads, but that seems to just detect correlation. I can't say "oh, this keyword gave me more signups, therefore I should use more money on it." I'm not sure of the simplest setup that lets me do that.