In the F/A-18C manual provided by ED, it says
From my own experience, and according to the NATOPS manual, pressing the ON/OFF button is not required, if ATTH, HSEL, BALT, RALT or CPL is desired, to enable autopilot. Selecting the A/P pushbutton and then selecting ON/OFF without colonising any AFCS mode engages the basic autopilot (heading hold). Engagement of AFCS modes only requires pressing the option pushbutton next to the option window displaying the desired AFCS mode.