Tracked links: /go and the fm parameter
What the firstmrr.app/go/<slug> redirect does, and how your code reads the link a visitor came through.
The redirect
https://firstmrr.app/go/<slug>PublicA slug is 3 to 30 characters (^[a-z0-9-]{3,30}$). The redirect answers 302 at once and counts the click afterwards, without any IP address; robots and link previews don't count. It adds the origin to your destination, never overwriting what you already put there:
https://example.com/pricing?utm_source=tiktok&utm_medium=social&utm_campaign=tiktok-oscar-3f9a&fm=tiktok-oscar-3f9aOn a site, the script reads fm on arrival: source() then returns { method: "link", ref: "<slug>" }. In an app opened by a link, save the value of fm as the fm_ref attribute. On the App Store, ct carries the slug: that campaign's downloads are tied to the link when your App Store Connect connection reads the detailed report.
What a link tracks
When creating it, in Distribution → Tracking, you can say what the link tracks: one of your connected accounts, a format, or "other" with a label. Allowed destinations are your connected site (and its subdomains) and App Store or Google Play pages: /go is not an open redirect.