Thank you for reaching out to the API Support team here at Constant Contact. That is a great question.
This can be accomplished by having a page you control which would be used as the redirect. You can then append a query to the redirect URI on your request which your site can interpret and forward the user to the proper domain along with all of the OAuth parameters.
For a more detailed response, please see this post here: https://community.constantcontact.com/t5/Developer-Support-ask-questions/Setting-up-authentication-b...
Please let me know if you have any other questions!
API Support Specialist
This information means that URL Query Parameters are not allowed within your registered redirect URI. You can use them in practice, they just can't be stored as a part of your registered redirect URI within Mashery.
Tier II API Support Engineer