You need to use:
await client.PostAsync(uri, content);
Something like that:
var comment = "hello world";
var questionId = 1;
var formContent = new FormUrlEncodedContent(new[]
{
new KeyValuePair<string, string>("comment", comment),
new KeyValuePair<string, string>("questionId", questionId)
});
var myHttpClient = new HttpClient();
var response = await myHttpClient.PostAsync(uri.ToString(), formContent);
And if you need to get the response after post, you should use:
var stringContent = await response.Content.ReadAsStringAsync();
Hope it helps ;)