Hi,
There is no such thing as a spec for Remote Follow, it is implementation-defined and is one of those things from GnuSocial/OStatus-era which didn’t get standardized into ActivityPub.
I would strongly suggest to not use Remote Follow or try to replicate it’s behavior as it works like phishing: Stating your account on another platform and hopefully being sent to your actual instance for the Follow.
Putting a link to the ActivityPub id, why not formatted like a webfinger address (which is an extension of the spec) should be the way to go, ActivityPub implementations tend to allows to paste the link into the search field.
Another other way it could be done would be via custom-schemes (ie. activitypub://follow/$id), sadly browsers are terrible and tend to not allow to pick handlers dynamically, good luck with WHATWG I guess.