I solved this issue, for people facing this, the redirect uri has to be given in this format in your custom OAuth2 server <your_application_client_domain>/oauth-authorized/<provider_name>
For example, If I create an OAuth2 provider dictionary entry called djangooauth in flask settings with all credentials given to it, the redirect uri to be given to the server while creating a client would be <client-domain>/oauth-authorized/djangooauth