As of Sites 9.6, you can also configure the CM Server to use "Access Management" authentication for desktop clients like Content Porter.
Technically speaking, this means that the (SOAP) Web Services used by those desktop clients accept OAuth 2.0 bearer tokens (obtained from Access Management). Furthermore, they expose an additional endpoint which reveals the Access Management URL to the client.
You can check whether your CM has been properly configured by accessing these endpoints in your browser. For example:
<YourCMBaseURL>/webservices/access-management-url. This should return a plain text response with the Access Management URL.
This, in turn, triggers the desktop client to start a user authentication flow in a browser and thus obtain a user Access Token. The user Access Token is then included as OAuth bearer token in all subsequent request for the Web Service.