You can right-click on any parameter knob of a VST Instruments GUI, i.e. Omnisphere's Filter Cutoff knob, then select Enable Host Automation from the drop down menu that appear when you right-click the parameter knob.
That knob's name will now appear in the VST Controller list of the VST instrument track.
Then you just .... Have fun Automating
Cheers,
Muziksculp