@Haibert Barfian Thanks for posting in our Q&A.
For this issue, I have done the test in my lab. I click on "F12" to open the Devtool to get the specific request.
When I click on "Assign user" to assign a user to the autopilot device, I will get the request URL. From the request, we can see the specific windowsAutopilotDeviceIdentityId.
And in my test, my graph api is not assignUserToDevice, it is UpdateDeviceProperties.
Then click on "Payload > view source", we can get the request body.
So, the request body is correct. The important is to find the "windowsAutopilotDeviceIdentityId".
Hope it will give you some ideas.
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