IOS ASWebAuthenticationSession for OAuth

Previously I used a WKWebView  browser control to perform the OAUth. This allowed me to trap the redirect URL to obtain the access code. My redirect URL was http://localhost/Auth

Now I am using the Native browser interface ASWebAuthenticationSession This requires that you use an APPLICATION specific callback URL scheme for your redirect uri. i.e. MyAppName://Auth

The Dropbox APP console allows me to specifiy this as the Redirect URL but gives an error when I try to call the  oauth2/authrize endpoint using this as the redirect_uri