You need the control room for this. Step by step:
Vist VST connections, enable CR.
Go to Outputs tab and disconnect your main out there.
Go to Control Room tab and rightclick to create a monitor output (reconnect your main outputs here).
Same page: rightclick to add a mono Cue channel, name it ‘Click’ if you like.
Connect the click-cue with any free mono output of your interface.
Connect that interface output physically with Mix B on your headphone amp.
Controlling the click:
Open the control room in the mixer. That thing is merged with the metering to the right.
You will find your Cue send there now, enable the click there. You can also control the level there or just leave it at zero as your HP preamp has hardware controls for it.