I followed this example https://docs.microsoft.com/en-us/azure/active-directory-b2c/active-directory-b2c-devquickstarts-web-dotnet-susi from the Azure AD B2C documentation page
The MSAL library (which is being used to exchange the code for the token in the sample) automatically calls the Azure AD B2C endpoint to exchange the refresh_token for a new access_token. You shouldn't need to deal with it.