SSO URL redirects user to deprecated portal

Hello,

When I use REST Api to get SSO URL to redirect user to our developer portal, the returned URL takes user to the deprecated portal:

https://management.azure.com/subscriptions/{{subscriptionId}}/resourceGroups/{{resourceGroupName}}/providers/Microsoft.ApiManagement/service/{{serviceName}}/users/{{userId}}/generateSsoUrl?api-version=2021-08-01

This call used to work (i.e. it was returning the URL that takes user to the new developer portal) before we added a custom domain.
The Management API URL is still pointing to the default gateway, and there seem to be no way to change it to the custom domain.

What can be done to have the above call to return URL that logs user into the new developer portal?

Thank you