Just specifying HTTPS in the URI should do the trick.
httpClient.BaseAddress = new Uri("https://foobar.com/");
If the request works with HTTP but fails with HTTPS then this is most certainly a certificate issue. Make sure the caller trusts the certificate issuer and that the certificate is not expired. A quick and easy way to check that is to try making the query in a browser.
You also may want to check on the server (if it's yours and / or if you can) that it is set to serve HTTPS requests properly.