No special tool is required.

I used a C-spanner which has a hole at the end of the grip. I attached a digital fish scale to that hole with a cable, put the C-spanner on the adjuster nut and pulled on the scale with 16 kg as perpedicular as possible. As the hole to which the scale is attached is 15 cm away from the centre of the steering stem, I have effectively tightened the adjuster nut with ~ 2.5 kgm.