How to create HTTPS request on UnitTest class?

Question

I created a Unit Test class for testing a simple HTTPS request

[TestMethod]
public async Task TestMethod1()
{

    string destination = "...";
    string message = "..";
    string accountUsername = "..";
    string accountPassword = "..";
    string senderPhoneNumber = "...";

    HttpStatusCode response = await SMSProvider.Send(destination, message, accountUsername, accountPassword, senderPhoneNumber);

    Assert.AreEqual(HttpStatusCode.GatewayTimeout, response);
}

And the SMS Provider is a Library class

public static async Task<HttpStatusCode> Send(String destination, String message, String accountUsername, String accountPassword, String senderPhoneNumber)
{
    HttpClient client = new HttpClient();

    var values = new Dictionary<string, string>
    {
        { "username", accountUsername },
        { "password", accountPassword }, 
        { "src", senderPhoneNumber },
        { "dst", destination },
        { "text", message },
        { "srr", "3" }
    };

    var content = new FormUrlEncodedContent(values);

    HttpStatusCode responseStatus = HttpStatusCode.Accepted;

    try
    {
        var response = await client.PostAsync("https:SPECIFIC-URL-HERE", content);
        var responseString = await response.Content.ReadAsStringAsync();
        responseStatus = response.StatusCode;
    }
    catch (WebException we)
    {
        string error = we.ToString();
    }
    catch (Exception e)
    {
        string error = e.ToString();
    }


    return responseStatus;
}

However it throws the following error

System.Net.Http.HttpRequestException: An error occurred while sending the request. ---> System.Net.WebException: The request was aborted: Could not create SSL/TLS secure channel.

How can i send an HTTPS request? Do I need a certificate or something?