Authentication Error while trying to get the DHL Interface to run

Question

I've been trying to get the DHL interface up and running for some time now.

Unfortunately, there is already a lack of authentication.

Request:

client_id == AppID
client_secret = AppToken

    var client = new RestClient("https://api.dhlecs.com/auth/v4/accesstoken");
    
    client.Timeout = -1;
    
    var request = new RestRequest(Method.POST);
    
    request.AddHeader("Content-Type", "application/x-www-form-urlencoded");
    request.AddParameter("grant_type", "client_credentials");
    request.AddParameter("client_id", client_id);
    request.AddParameter("client_secret", client_secret);
    
    IRestResponse response = client.Execute(request);
    
    Console.WriteLine(response.Content);

Response:

{"type":"https://api.dhlecs.com/docs/errors/401.0000007", "title":"Invalid credentials"}

Does anyone have any idea why it doesn't work? Or am I calling something wrong?