HttpClient PostAsync Error

See more:

ASP

.NET

MVC

REST

If am using below code to get authentication response from service.

using (var client = new HttpClient())
{
	client.BaseAddress = new Uri
	var content = new FormUrlEncodedContent(new[] 
	{
		new KeyValuePair<string,>("username", _username),
		new KeyValuePair<string,>("password", _password)
	});
	var response = client.PostAsync("api/authorize", content).Result;
}

However getting aggregate exception and inside it, below exceptions are given:
{"An error occurred while sending the request."}
{"Unable to connect to the remote server"}
{"No connection could be made because the target machine actively refused it 216.167.113.4:443"}

Have gone through some of the forums and one of the possible reason could be Firewall ( on which i have no rights to put it on in OFF mode)
But if I am able to access the same service through browser I am able to access.
So not sure whether its a firewall problem or not. Can anyone please suggest or guide me, how can I be sure of the root cause of this problem.

Struggling a lot with this issue, please help me out