Cannot connect ot Celestron 127SLT telescope

When I plug in my Celestron 127SLT telescope into my laptop via this adapter and the Celestron serial cable, what “port” do I select in the nexstar observer list software to make it control the telescope? When I try to connect my laptop to the telescope, it says “unable to communicate with scope, invalid port number”, or no connection to the port.
Thanks!
:slight_smile: