It might not work like this in other clients but in amethyst if I have the npub of the person I want to tag I just put the @ followed by the npub and the handle populates. If I know the handle and I add the @ and start typing it, sometimes it populates. If it doesn't then I go to nostr.band and search the handle to find the npub to add it to my note... Not the most straightforward workflow but that's how I do it. I would be curious if anyone else has a easy way too.