You should be able to use one of the inputs on the HG08 for the spindle pulse.
The manual is a bit vague, but I'd guess they'll be using the same high speed optocouplers as the outputs, so should be able to handle an index pulse.
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Spindle speed control will require some kind of external step to analogue converter. One thing you might not realise about the HG08, is even though the connectors are labled for specific axis, you don't have to use them for that purpose. The step and dir pins on each axis connector can be configured and used as a normal output.