Add specular to only a specific part of makeup?

Hello! I'm trying to finish this eyeshadow I'm making and have been trying to add a specular to make the glitter shine the way I'm envisioning. The issue is, when I add the glitter as the specular it adds the shine to the entire eyeshadow, ass opposed to just the glittery part. Is there any way to make it so that it's exclusively on one part of the eyeshadow?