Hoxton mask install:
1. Open OpenIV
2. Navigate to GTA V/x64v.rpf/models/cdimages/streamedpeds_player.rpf
3. Depending on which character you want to add a mask for, either go into player_zero, player_one, or player_two, and place the modded file.

Please note that to actually wear this, you have to go to the mask merchant or use a trainer of some sort that allows you to customize your characters model/skin.(https://de.gta5-hub.com/scripts/menyoo-pc-sp)