Here is my two cents on the subject. I've had several dry orgasms which feel incredible, but never a full wet orgasm, but I haven't really been trying to, just enjoying the trances I've had.
You need to be safe, comfortable, and free from distraction. No clothes, make sure you're warm. No blankets if you can help it, as others have said above.
You want to feel like you are floating freely in the words and suggestions of the hypnotist, not the weight of clothes/blankets.
I find it hard to let go of the day so maybe try 5-10mins of some meditation to let your mind settle before going into trance.
Just let it what happens, happen. Don't force it, don't think "I have to have a HFO this trance". You're too focused on it. Focus on the words, on the feeilngs. Visualize yourself laying there and reacting how you would think you'd react to the stimulation.
Once you can start to lose yourself and float into trance you'll see more results from "doing nothing". To me that's the hardest part. Just being able to mentally let go and let the hypnotist take me where they want and I respond accordingly
I think it helps if you are listening to sessions that build up over time. I've never listened to one session and had a dry or wet orgasm. It takes time to buildup.
Also stop masturbating, drink lots of water. You want your body ready to and waiting to cum. You could try edging earlier in the day to get that feeling in your mind.
Practice doing kegel exercises. It will help in all aspects of your sexual life, but it will really help that "clenching" feeling that you should start automatically having when you get more and more aroused.
You don't need a raging hard on for an hour to have an HFO. It's more about the feeling in you mind driving your body. Don't get it backwards. Plus you'll probably lose your erection sometime in the trance and think "Shit no can it happen now" which is untrue. Just let your mind go and whatever happens physically happens.
Overall stop focusing on the orgasm, and focus on the experience of getting there. The end result shouldn't be the goal, just the icing on the cake.
Hope it helps!