Getting errror missing-input-secret

In my asp.net c# application after submit I keep getting error:

{{
“success”: false,
“error-codes”: [
“missing-input-secret”
],
“messages”:
}}
}

I have double check my secret key , but not sure why it keeps failing.

   string token = formCollection.Get("__RequestVerificationToken");
          

            var formData = new MultipartFormDataContent();
            const string SECRET_KEY = "0x4A..........";

            //formData.Add(new StringContent(SECRET_KEY), "secret");
            //formData.Add(new StringContent(token), "response");
            formData.Add(new StringContent(SECRET_KEY, Encoding.UTF8, "application/x-www-form-urlencoded"), "secret");
            formData.Add(new StringContent(token, Encoding.UTF8, "application/x-www-form-urlencoded"), "response");

            var url = @"https://challenges.cloudflare.com/turnstile/v0/siteverify";
            var client = new HttpClient();

           
            var response = await client.PostAsJsonAsync(url, formData);
            //PostAsJsonAsync
            var returnValue = JObject.Parse(response.Content.ReadAsStringAsync().Result);