I am using DropBox - I am passing on what I learned in logging into DropBox.
DropBox is found at "https://www.dropbox.com". The DropBox developer site suggests that "https://api.dropbox.com" be used for developers. They are different sub-domains, which track their credentials separately.
That means that if authorization starts at the 'www' subdomain, then your AuthorizationCallBack should be 'https://www.dropbox.com/home#:::' (the home page for your email/password combo).
When I first setup SharpBox for DropBox access, I set the AuthorizationCallBack equal to 'https://api.dropbox.com/home#:::', which meant that I logged in first to the 'www' sub-domain and then was prompted to log into the 'api' sub-domain.