1. First you have to create and save an outfit with the mask you plan on wearing with a mask.
2. Open up your interaction menu and set the bike helmet to the helmet you want to wear with the mask and set Auto Show Bike Helmet to on.
NOTE: If you want Deadline or Pilot helmets, equip a Deadline or Pilot outfit first them scroll through Bike Helmets until you reach the chosen helmet. You can then re-equip your outfit with the mask.
3. Get on a motorcycle. Open up the interaction menu and go to SecuroServ - Management - Style and scroll once to the right to the Founder outfit. You'll see your character will put on the chosen helmet. During this quickly scroll back to None and you will be wearing the helmet and mask.
4. Without getting off of the motorcycle, go to the Mask Store in Vespucci.
5. When you arrive, get off of the motorcycle and go to the Hats Stall. Scroll through briefly and back out. This will make the helmet stick with the mask.
6. Now you can save your outfit.