There is a step by step configuration to add API Management Administrators and then use a Powershell command to retrieve a URL to open the Developer portal as an administrator (https://docs.microsoft.com/en-us/azure/api-management/api-management-faq#how-do-i-add-a-user-to-the-administrators-group). But unfortunately, it is not working with the new developer portal, only with the legacy portal.
What are the steps to use with the new developer portal?