A quick an easy way to authenticate on Azure management API is to use az cli get-access-token command and use this as a Bearer token on your API calls.
Given that you want to access https://management.azure.com/subscriptions/api you can use:
$token = az account get-access-token | ConvertFrom-Json
You can then get the access token with $token.accesstoken
Finally you can use this token as a Bearer token on your API calls
$headers = @{ Authorization = "Bearer $token.accesstoken" }
