I do not think you would need one because with SAML the identity provider handles the authorization. Assumption is that both in Mendix and in Docusign you have setup the SAML integration. As soon as Mendix redirects to Docusign then DocuSign will do the check if the user (which is known because the userId is handed over to DocuSign) can have access to that application.
So the reason you do not need the endpoint is because both applications (Mendix and DocuSign) already know the endpoint ie the server where to sent the SAML request.
Regards,
Ronald