You can connect a pot, which if you have one laying around (IIRC they're normally around 100k), I would wire up just to check the drive still works. But as I've already mentioned, be aware those control terminals are probably around half mains voltage.

However I'd aim to keep the spindle computer controlled.
Most breakout boards are capable of 0-10V control, and just need an isolated 12V power source to avoid issues with non-isolated drives. A small DCDC converter is typically used for this, but that power board may already provide an isolated supply.

Inhibits on those boards generally need connected to inhibit. In the case of H, it needs power applied, or in the case of L needs connected to 0V (which may not be 0V in relation to earth)