As per this post, I'm wanting to generate usernames for new players.
The obvious solution is to create a username based on how many players are registered, so I can call on the client...
string newUsername = CloudScript("GenerateUsername");
RegisterPlayFabUser { Email = email, Password = password, TitleId = "xxxxxx", Username = newUsername };
This isn't supported and a client has to be logged in to call CloudScript, meaning we can't use PlayFab server-side code to help with user management prior to registering.