System.Net.HttpClient.PostAsync work in UWP, but not working in .Net Standard Library - win-universal-app

Question

a have UWP app, that working correctly. Now i working on .Net Standard Library providing same function and i do not now, what I'm doing wrong.
UWP code (working correctly)
private async void btnRVM_ClickR(object sender, RoutedEventArgs e)
{
using (HttpClient httpClient = new HttpClient())
{
Uri uri = new Uri("http://examplecom/api/Account/Register");
HttpContent content = new StringContent(JsonConvert.SerializeObject(new RegistrationObject(tbxUsername.Text, tbxPassword.Text, tbxConfirm.Text)), Encoding.UTF8, "application/json");
try
{
HttpResponseMessage response = await httpClient.PostAsync(uri, content);
}
catch (Exception e1)
{
string excMessage = e1.ToString();
}
}
But almost same code in .Net standard library do not working
public static async Task<object> RegisterAsync(string username, string password, string confirmPassword)
{
using (HttpClient httpClient = new HttpClient())
{
Uri uri = new Uri("http://example/api/Account/Register");
HttpContent content = new StringContent(JsonConvert.SerializeObject(new RegistrationObject(username, password, confirmPassword)), Encoding.UTF8, "application/json");
try
{
HttpResponseMessage response = await httpClient.PostAsync(uri, content);
}
catch (Exception e1)
{
string excMessage = e1.ToString();
}
return Task.FromResult<object>(null);
}
}