The coupling I have is a bending coupling. This coupling is not appropriate because I need a rigid and well-fixed shaft.
So I plan to make a rigid coupling with this belt gear fixed shaft.
Drill the inner diameter of the coupling to 6mm to fit the motor shaft, and make a 3mm hole in the side tube. This 3mm hole is the space for the fixing screw of the shaft and extension shaft of 4mm diameter.
Then it is complete.