FA doesn't tell you it very clearly (well it does but it's so easy to overlook it might as well not be there) but when filling in the contact fields you only have to put in the username and the field will autofill the rest of the link (with the exception of Mastodon and personal website links), otherwise filling in the full link completely breaks the link as the other peep explained โ๏ธ
*edit* ah it works on the contact field down below but not the link in your profile