You can use your virtual keyboard. Which DAW are you in?
You have to map the input of your MIDI Instrument track to 'listen' for the notes from your internal virtual keyboard.
Reaper - Setting up Midi Input - Recording and Monitoring - Untidy Music
I'm not a Reaper user and haven't even looked at it in years but the concepts are the same for most DAWs.
Make a new MIDI Track.
Set its Input.
This says Right Click on the AR button, but I also see an 'IO' (Input/Output) button so my guess is they have the same options in regards to Input.
Choose 'Input MIDI >' and I see something in their drop list that says 'Virtual MIDI Keyboard.'
Do I need to have a USB to midi in from the laptop to the rack synth and then an audio cable back to the laptop?