test.cs

private async Task RequestCodeAsync()
{
    string clientId = "";
    string serviceUrl = "https://trial.dsserver.io";

    HttpClient client = new HttpClient();

    var queryParams = new Dictionary<string, string>
    {
        ["response_type"] = "code",
        ["scope"] = "signature",
        ["client_id"] = clientId,
        ["redirect_uri"] = "https://localhost:7194/Home/Token",
        ["state"] = "teststate",
    };

    string requestUri = $"{serviceUrl}/oauth/authorize?{UrlEncode(queryParams)}";

    var codeRequest = new HttpRequestMessage(HttpMethod.Get, requestUri);

    await client.SendAsync(codeRequest);
}