I taught my GSD to "hold" as part of her retrieve exercise.
When I first started, I would just touch her muzzle with one pointer finger, say "hold," mark and treat, then touch with both pointer fingers (one above/one below her muzzle), mark and treat. Apply a little pressure with both fingers, mark and treat.
Over time, I would open her mouth (without an object), mark and treat. It eventually led up to me putting an object in her mouth, closing her mouth, with my pointer fingers applying gentle pressure, say "hold," mark and treat. If she tries to chew it, I just gave her a negative marker, then mark and treat when she holds it for a few seconds without chewing.