Moisture in the air alone won't help. You have to find a way to breath through the nose instead of mouth breathing. Some people use chinstraps to keep their mouth from opening involuntarily and some even use special mouth tapes. I wouldn't recommend mouth tape but a chinstrap may be worth a try

For the neighbouring problems this thread keeps touching, the shelf has: CPAP Dry Mouth and Mouth Breathing: How to Stop It Tonight — dry mouth, and why it is so often a mouth-leak symptom. CPAP Mask Leaks & Mouth Leak: Causes, the Data Threshold, and Fixes — the leak checklist in order. CPAP Nasal Congestion and Stuffy Nose: Causes and Fixes — the opposite complaint, congestion on therapy. Troubleshooting & Optimizing CPAP — the overview topic for the whole room. Comfort settings are yours to adjust; pressure is not. If comfort problems are pushing you towards changing prescribed settings, that is the moment to talk to somebody rather than to the menu. Nothing on SomniTalk is medical advice. Talk to your own clinician before changing therapy.

Reading, for the three separate problems this thread braids together: CPAP Dry Mouth and Mouth Breathing: How to Stop It Tonight — the dryness itself. CPAP Mask Leaks & Mouth Leak: Causes, the Data Threshold, and Fixes — the leak checklist, in order. CPAP Nasal Congestion and Stuffy Nose: Causes and Fixes — for when the nose is part of the story. Nothing on SomniTalk is medical advice. Talk to your own clinician before changing therapy.