I Am A Real Werewolf
Uh, not to ruin anyone's fun (or placebo effect), but shifting charms aren't really a thing, nor are spells to stop or start shifting. While these things can work through the magic of placebo effect, they generally do not have a legitimate effect on shifting, and there is no charm that is going to start or withhold shifting so you can stay in your form. Nor is there a spell that can do the same, and generally magic like that is so powerful as to be rare-you're not going to find a 50 year old spell for making someone shift on YouTube, nor are you going to be able to make one yourself. It's best to let these things come naturally.