Lifting heavy and minimizing strap use will force your forearms to grow without direct work. However, let's say your forearms are severely lacking and you want to target them with specific exercises. Then include supersets which facilitate continuous tension. Wrist curls for the flexor muscles are a good option.
For whatever reason, curling behind your back feels like a better position for targeting these muscles. Cables tend to facilitate the continuous tension so begin with behind-the-back wrist curls with a straight bar for 10-12 reps.
Immediately cut the weight in half, turn around, and with a pronated grip (palms down) do reverse curls. The reverse curls will hit the biceps but also the brachioradialis. Keep rest breaks at a minimum and perform this superset for 4 rounds.