Thanks for reaching out.
Since you mentioned request_token, it looks like you are using the OAuth 1.0a protocol. You won't find a field to configure this on our website because OAuth 1.0a does not rely on a pre-configured callback URL in your dashboard.
Instead, you must dynamically specify the URL by including the oauth_callback parameter in your request.
For OAuth 1.0a: Add the parameter to your call like this (ensure it is URL-encoded): oauth_callback=https%3A%2F%2Fyourapp.com%2Fcallback
A Note on OAuth 2.0: If you decide to upgrade to OAuth 2.0 in the future, the process changes. The redirect_uri must be hard-coded on our end.