It used to be a combination of your screen name AND number tag together, but they’ve changed it recently so it’s now just a unique username. I’m not really sure if the friending system has changed yet.
Adding someone with a “legacy username” (AKA combination of screen name and number tag), you’ll need to type them both, one right after the other. So for example “BobSmith#6789”, where “BobSmith” is the person’s username and the number tag is the hashtag. There can be multiple users named “BobSmith” but only one “BobSmith#6789”.
Hope this helps.