OnlineSubSystem login with name/password from custom UI

I’m implement my custom OnlineSubSystem plugin. But I meet a problem, there is no way to input name/password from blueprint.
The old “ConnectToService” will call OnlineIdentity->Login without any name/password can input.
The new “ShowLoginUI” is totally undocumented. I guess this is the way to do in game input. But this will limit the UI style.
Is there any exist blueprint script entry can do a login with name/password?