Where to put redirect URL after authentication?

Fairly new to the .net architecture and trying to incorporate the dropbox API into our webapp - we have OAuth set up, and currently we are sending both antiforgerytokens and the redirect URL in the 'state' parameter (as seen here) however the encrypted state we are sending becomes too long as the redirect URL is long to begin with - was wondering if there's another way to send the redirect URL (not the redirect URI which is generated by the app...) via the API so the user is able to go back to our web app after authentication?

Also from the embedded link above: the "redirect_uri String? Where to redirect the user after authorization has completed. " sentence is kind of misleading, almost as though this is the parameter im looking for but its asking for the Dropbox generated URI and not the redirect url (where to send the user after auth).

Any help is much appreciated, thanks.