First time trying async
There isn't much to review here, but I would probably have introduced a variable for the awaited call's result, and then used a ternary to determine the outputString; the difference would have been that the awaited call would have stood out more, but there's nothing really wrong with the way you did it.

That said outputString would probably be better off as outputMessage, or simply output - having the type name in an identifier is never a sign of great naming.

I realize it's a tiny little example, but you have everything done in the UI's code-behind here - that's only good for prototype/throw-away code (which this probably is). In a real app you would have coded the logic elsewhere.