"Missing redirect_uri parameter" error with Xamarin.Forms UWP sample

[ThaineNorris]

I was able to get the proper Facebook login flow with the Sample.Forms.UWP sample app. I get the "Success! User is logged in" message, but it stops there and I see exceptions in the debug output. I traced it down to line 229 in the RefreshToken function of OAuthApi.cs where Facebook returns an error that reads,

error :
message : Missing redirect_uri parameter.
type : OAuthException
code : 191

Then I think that since RefreshToken does not succeed, the login is not actually saved on the local machine, so the user must log in again every time the app is run.

How do I supply a redirect_uri? Or is that really the problem?