Control the speed of a 24 V DC motor with extra control voltage wire with Arduin

No I have not. I think that could be the missing piece.
So that means I have to connect the black wire (- Supply voltage, GND) with the minus of the DC power supply (already done) AND the GND of the Arduino (the one next to digital pin 13)?

Thanks!

I attached the drawing. I am not familiar with those drawings though, sorry. The dashed line is the missing connection, I think.