It would seem you need to pass a client id also as I receive the following message when using your url:
SurveyMonkey
The authorization request failed: Missing required parameter(s) redirect_uri and/or client_id.
When adding a random client id (e.g. https://api.surveymonkey.net/oauth/authorize?redirect_uri=notsureyet.com&client_id=xx ) the following message is displayed:
SurveyMonkey
The authorization request failed: Invalid client_id "xx".