Which endpoint are you trying to use, it's not clear from your example? If you are trying to use the /security/alerts_v2 one, that's indeed the expected behavior, no POST method therein.
If the idea is to generate some sort of notification when a specific user is logging in, you can use Activity alerts/Defender for Cloud apps activity policies: https://learn.microsoft.com/en-us/defender-cloud-apps/control-cloud-apps-with-policies