Below is example to call synchronously but you can easily change to async by using await-sync:
var pairs = new List<KeyValuePair<string, string>>
{
new KeyValuePair<string, string>("login", "abc")
};
var content = new FormUrlEncodedContent(pairs);
var client = new HttpClient {BaseAddress = new Uri("http://localhost:6740")};
// call sync
var response = client.PostAsync("/api/membership/exist", content).Result;
if (response.IsSuccessStatusCode)
{
}