Don't even think about your wrist when serving.
Grip the racket with a mediun tension that allows your wrist a little bit of flex. Because of the forces involved, you wrist will never stay perfectly stiff, but will rock just a bit like a sort of hinge.
As your racket strikes the ball, if your timing is good, your wrist will be pretty much straight at impact. At the point of impact, your forward motion is checked and there is a SLIGHT flexing that is a part of the total smooth flow of motions that make up the serve.
Do not try to force anything. As much as possible, keep the action loose and fluid.
Good luck,
B