/d2l/api/lp/1.3/users/?orgDefinedId=[our org defined ID]
/d2l/api/le/1.3/auditing/auditors/[D2LID for the auditor]/auditees/
{ "AuditorID" :[D2LID for the auditee] }
// Create Auditor Relationship
var request_create = new RestRequest("/d2l/api/le/1.3/auditing/auditors/" + auditor_d2lid + "/auditees/", Method.POST);
request_create.RequestFormat = DataFormat.Json;
request_create.AddBody(new { AuditeeId=auditee_d2lid });
auth.Authenticate(client, request_create);
var response_create = client.Execute(request_create);
Console.WriteLine("Auditor Relationship Creation Status: " + response_create.StatusCode);
Console.ReadKey();
It looks like you're sending the AuditeeId in a format the API doesn't expect. Basically, the route is expecting a JSON number, rather than a more complex JSON object. There's a related question on StackOverflow where another Valence Community Member did the same thing with a different call: http://stackoverflow.com/questions/21444939/what-is-the-expected-json-block-for-a-desire2learn-post-org-parent. Take a look at the response on StackOverflow for details, and let me know if that resolves the issue for you.
// Create Auditor Relationship
var request_create = new RestRequest("/d2l/api/le/1.3/auditing/auditors/" + auditor_d2lid + "/auditees/", Method.POST);
request_create.RequestFormat = DataFormat.Json;
request_create.AddBody(auditee_d2lid);
auth.Authenticate(client, request_create);
var response_create = client.Execute(request_create);
Console.WriteLine("Auditor Relationship Creation Status: " + response_create.StatusCode);
Console.ReadKey();