Sign API Authentication | Refresh token Url

We are working with the Adobe Sign API to integrate that with our system. Currently, Sign API only supports the OAuth 2.0 Authorization code grant. Our system SDK will use the Token URL endpoint for refreshing the Access token. Since the refresh token URL is a different endpoint than the Token URL we can't refresh it from our system SDK. Is there any other supported authentication for Adobe Sign or any possible solution to refresh the token using Token URL?