As described here (see also comments): https://community.playfab.com/questions/68177/link-email-and-password-to-anonymous-account.html?childToView=68338#comment-68338
it would be very convenient for the player to remove seemingly unnecessary condition to provide username in AddUsernamePasswordRequest. There should be another request - AddEmailPasswordRequest which does not require username.
As there is already register method that does not require username - RegisterPlayFavUserRequest, this should be possible.
