Hi piercleo,
It depends. If you control both websites, I would enable cross domain tracking. Here is link to how to do this in google analytics [UA] Set up cross-domain measurement (analytics.js) [Legacy] - Analytics Help. This will essentially link your two websites and look at the original source of sessions between both websites.
If you only have control over the landing page, or want to limit the link for just this page you will have to get into some javascript to fetch the URL variables and append them to the link on the CTA. Here is a link to some info about what that might look like.How To Get URL Parameters With JavaScript - The easiest way .