Need some help about getting 10V for my CNC spindle control

I need some help on the electrical side.
This is my CNC breakout board:
Purelogic PLCM-B1 english manual

Page 10
Connector XP79 (upper right) needs 10V and GND and outputs an opto-isolated 0-10V PWM signal to control the spindle RPM with.

From my USOVO STG104 spindle controller I only get 24V supply voltage and GND.
And it needs that 0-10V signal.

So I do a voltage-divider of 12Kohm and 16.8KOhm to get 10V.
As soon as I connect that to the PLCM-B1 breakout board my 10V drops down to 2.4V from the load.

What should I have done instead?
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