Yes, it is possible to be hypnotized even if you don't believe in it, but belief can affect how easily you enter a hypnotic state. Hypnosis largely depends on your level of suggestibility and willingness to cooperate. If you are completely resistant or skeptical to the point of refusing to engage with the process, hypnosis may be less effective or not work at all. However, some people who are initially doubtful still respond to hypnosis because it is more about focus and relaxation than belief.
In summary:
Belief is not absolutely necessary for hypnosis.
Openness and willingness help increase hypnotic susceptibility.
Extreme skepticism or resistance can reduce or block hypnosis.