Adding Hydraulic Fluid to Trim Pump

[tom from naperville]

I replaced my worn out trim motor today. When I disassembled it, I lost maybe 3/4 of a cup of hydraulic fluid. Trim works, but it seems slow, which I'm assuming is because it needs the fluid topped up. Now I need to know the proper procedure. It looks like there is a plastic cap on the side of the pump with a warning sticker next to it. Do I just open up this cap and pour in some more fluid... or?
This is an OMC 135HP Inboard, 1993 vintage.
Thanks,
Tom