It seems there is currently no way to query the user for a username (text) AND a password at once.
But this would be a very useful feature ... maybe just like a collection of buttons it should be also possible to use a collection of input fields. Or at least allow something like this:
inputMode = InputMode.Text | InputMode.Password;
And add an additional "Password" property to the result object besides the existing "Text" property. For backwards compatibility, you could still fill the "Text" property with the password if only InputMode.Password is requested.
Ideally, each TextBox could have a descriptive title too, but that could be achieved by a custom template too.