Neck buttons / neck brooch
If buttonholes are sewn into the shirt collar, you can use neck buttons. Many national costumes used to fasten the shirt to the collar in this way, but now many national costume shirts are sewn without buttonholes in the collar. Then you can use a neck pin. In any case, some kind of brooch is necessary to have; it's a "must-have" among the silver for any bunad. You would get some looks with a low-cut bunad shirt...
