I think it's typically hidden by UIs for privacy reasons, but you could see it via something like: curl --header 'accept: application/activity+json' --location <a href="https://sopuli.xyz/u/cyrus" rel="nofollow ugc" target="_blank">https://sopuli.xyz/u/cyrus</a> | jq -r .matrixUserId
or curl --header 'accept: application/json' '<a href="https://sopuli.xyz/api/v3/user?username=cyrus'" rel="nofollow ugc" target="_blank">https://sopuli.xyz/api/v3/user?username=cyrus'</a> | jq -r .person_view.person.matrix_user_id
As the other comment says, backends hook into Matrix if someone uses the 'Send Secure Message' option to contact you (I don't think that option shows if you don't both have a Matrix ID).