Are werewolves forced to shape shift at the full moon?

In what fantasy game or piece of fiction?

There are no real shape-shifting werewolves. Anyone writing about them is free to put whatever restrictions on their shape-shifting abilities they like. In some cases, the shape change may be voluntary at other times but involuntary at the time of the full moon. In others, they may never be able to voluntarily change, but only as a result of the moon's phase.

The situation you've stated is probably the most common situation in works that I've read or movies that I've seen or games that I've played, but it's not actually mandatory, since there are no real werewolves to prove a writer incorrect.