Android Management API Setting Service Credentials

Question

I'm trying to get the Android Management API working. I'm using the C# library available here https://www.nuget.org/packages/Google.Apis.AndroidManagement.v1

I've generated a service account and downloaded the json key for that service account. Then used the code below.

  var client = new AndroidManagementService();

  var signUpUrlsCreateRequest = client.SignupUrls.Create();

  signUpUrlsCreateRequest.CallbackUrl = "https://localhost:5005";
  signUpUrlsCreateRequest.ProjectId = "rms";

  var json = "\\Downloads\\rms-c5ac188362d4.json";
  var stream = new FileStream(json, FileMode.Open, FileAccess.Read);
  var credentials = ServiceAccountCredential.FromServiceAccountData(stream);
  signUpUrlsCreateRequest.Credential = credentials;

  var result = signUpUrlsCreateRequest.Execute();

I'm getting the following error

Request had invalid authentication credentials. Expected OAuth 2 access token, login cookie or other valid authentication credential. See https://developers.google.com/identity/sign-in/web/devconsole-project. [401] Errors [ Message[Invalid Credentials] Location[Authorization - header] Reason[authError] Domain[global] ]

Any ideas of what I'm missing to get this to work?

Thank you