I read a Frederic Delavier’s book « Strength Training Anatomy » and there is good stuff.
Sitting with your forearms on your thighs or on a bench. Hold the barbell with a supinated grip :
Inhale and flex your wrists
Exhale at the end of the movement
This exercise work your flexor carpi radialis muscle, palmaris longus muscle, flexor carpi ulnaris muscle, flexor digitorum profundus muscle and flexor digitorum superficialis muscle.
The two last muscles constitute the essential part of the flexors volume of the wrists.