ارسال اطلاعات به API در Body

private const string TOKEN_ADDRESS = "http://localhost:52888/token";
using (WebClient client = new WebClient())
{
    NameValueCollection postParameters = new NameValueCollection();
    postParameters.Add("grant_type", "password");
    postParameters.Add("username", "user name");
    postParameters.Add("password", "password");

    var response = client.UploadValues(TOKEN_ADDRESS, postParameters);
    txtToken.Text = Encoding.UTF8.GetString(response);
}