Once you are listed in the json on his domain you have to update your profile at a site like https://metadata.nostr.com (after installing nos2x extension) that links your account to the domain json for verification
Discussion
You can look at domain.com/.well-known/nostr.json for your username to pub key mapping and then in NIP05 field you’d put username@domain.com (replace domain.com with nostrplebs.com I think it was in above)