It is most definitely possible to be hypnotized. It's also very much possible to be hypnotized long-distance. The only real thing with long-distance hypnosis is that every phase of separation means one less tool to work with. If you're not in person, you can't use touch as a tool, but video call gives you audio, visual, and verbal tools to work with. Audio call or audio track limits you to verbal and audio tools, while text limits you to verbal tools. (I've been hypnotized in person, over video chat, over audio call, by audio files, and via text.)
What you need to be hypnotized will be whatever you need. Every individual is unique when it comes to what methods, tools, and styles work well for them.
If hypnosis is something you're interested in, but you don't want to get involved just yet, a "warm-up exercise" you can do is to practice using your imagination to conjure up a scene, and try to imagine it as real as possible. That can also potentially help you uncover your modal (audial, visual, kinesthetic (touch), etc.), your modal being what kind of cues you to respond to best.
In terms of reading resources, Chloe mentioned some here >>6006257
and there's also Wiseman's book, "Mind Play" which focuses more on casual hypnosis for sexy funtimes.
If your dom is hypnotizing you, he'll need to learn and practice the techniques, which often involves a bit of wordsmithing.
Also, as Ghost >>6006734
already mentioned, practice safety, and make sure you have a way to remove triggers before installing any.