Thanks for reaching out about adding registrants to your meetings! To clarify, if successful, our Add Meeting Registrants endpoints should return a 201 response.
Here is an example request in C# — let me know if this helps:
var client = new RestClient("https://api.zoom.us/v2/meetings/{meetingId}/registrants");
client.Timeout = -1;
var request = new RestRequest(Method.POST);
request.AddHeader("Content-Type", "application/json");
request.AddHeader("Authorization", "Bearer {token}");
request.AddParameter("application/json", "{\n \"email\":\"someone@example.com\",\n \"first_name\": \"Someone\",\n \"last_name\": \"Somewhere\"\n}", ParameterType.RequestBody);
IRestResponse response = client.Execute(request);
Console.WriteLine(response.Content);