"Manual Page 445, Chapter 12.7.3
Special cursor functions in TGP can be used in both cockpit by pressing the PB 7.The
WSO can additionally select them by pressing the PB 7 or by moving the Castle
Switch (see Rear Cockpit HOTAS functions earlier in this chapter). To perform the
special cursor function:
In the front cockpit, TDC should be pressed.
In the rear cockpit, HC Trigger should be pressed to full action.
The possible options are: blank (no function selected), TGT (target), UPDT
(update), CUE and MARK."
You need to be in TGT at PB 7 to designate a target. If it is blank or another mode, press PB 7 until TGT shows above PB 7.