Certain 3rd party domains must not be used in Google Ads. For example, let us say I have an Upwork profile or an Upwork gig. If I use the Upwork domain in Google Ads, in an attempt to advertise my Upwork gig, no matter if I try a million times, the (Google) ad will get disapproved or most probably the Google Ads account will get suspended.
And "third-party registration link" would obviously be an affiliate link or a referral link : CANNOT be directly advertised in Google Ads : The (Google) ad will get disapproved or most probably the Google Ads account will get suspended.
For what cannot be done, why have you been "researching" for 4 to 5 months ? Can someone teach you ? Buy a domain and create a landing page. Let the landing page be informative in nature (unique content) and you can place a relevant third-party registration link on that landing page. It is a simple as that. If not, you will keep remaining in your "same situation".