No, you can't let it bounce.
No, you don't have to release it from the hand.
World Squash rules (effective at January 2019) state in section 5 The Serve,
5.7 A serve is good, if:
5.7.1 the server drops or throws the ball from a hand or racket and strikes it correctly on a first or further attempt before it touches anything else;
"Anything else" ...