Well, I'll take a stab. I would make sure any hydraulic cylinders attached to it are retracted, and fill it up at least to the base of your filler neck, where it 90*'s out the side of the clutch housing.
Now, I will assume you know that the M did not come factory with live hydraulics. There were kits to add it--I think Char-Lynn had one, you could put various IH parts together to make it happen, and M&W made a kit that was probably the most popular. Most of them provided for dropping the standard M belly pump/housing, removing the pump, fabbing up a cover plate and using the old housing for a reservoir. You had to drill and tap a 3/4" pipe thread hole near the bottom of the reservoir for your suction line.
Forget what I just said if you already knew all that....