According to this article on Chabad.org,
I don’t think there is a Jewish version of crossing fingers. You could try twisting them into a Star of David, but that is more likely to bring arthritis than good luck. Besides, we don’t believe that good fortune comes from signs and gestures. We pray to G‑d, do good deeds and have faith in the future.
The language we use shapes the way we think. So rather than say “I’m keeping my fingers crossed that I’ll get the job,” say “If G‑d wills it, I’ll get the job.” If it’s not meant to be, no finger contortion can change that. And if it is G‑d’s will, no “witch” can get in the way.
This article states that it is permitted to use the (Hebrew) expression מחזיק באצבע, meaning crossing fingers, but that it would be better to use a more Jewish alternative, such as מחזיק בימינך.