test.cs

private string GetAccessTokenClientCredentials()
{
    // Security credentials
    string clientId = "";
    string clientSecret = "";
    string serviceUrl = "https://trial.dsserver.io";
    string clientCredentials = "client_credentials";

    BearerToken token;
    HttpClient client = new HttpClient();

    // Generate the payload
    var payload = new Dictionary<string, string>
    {
        ["grant_type"] = clientCredentials,
    };

    // Token endpoint
    string requestUri = $"{serviceUrl}/oauth/token";

    // Create the request message
    var tokenRequest = new HttpRequestMessage(HttpMethod.Post, requestUri)
    {
        Content = new StringContent(
            UrlEncode(payload),
            Encoding.UTF8,
            "application/x-www-form-urlencoded")
    };

    // Add basic auth header containing client id and secret
    string credentials = $"{clientId}:{clientSecret}";
    byte[] credentialsUtf8 = Encoding.UTF8.GetBytes(credentials);
    string credentialsB64 = Convert.ToBase64String(credentialsUtf8);
    tokenRequest.Headers.Authorization =
        new AuthenticationHeaderValue("Basic", credentialsB64);

    // Send the request
    var tokenResponse = client.SendAsync(tokenRequest).Result;

    // Retrieve and return the token
    var tokenResponseStream = tokenResponse.Content.ReadAsStringAsync().Result;
    token = JsonConvert.DeserializeObject<BearerToken>(tokenResponseStream);

    return token.AccessToken;
}