Omamori has nothing inside

Hello!

I received an omamori from a Mochi place in Australia that was doing a promotional thing for its grand opening in a new spot. When I removed it from its plastic casing, it fell open since it was not tied shut.

After some googling, I've discovered you're not supposed to look inside of them. The thing is, it's empty. There's no prayer paper, no wood, no coin, nothing.

Does this mean it's just a trinket? Now that I know what an omamori is, I'd like to be able to use it. Am I allowed to put a prayer in it? The workers had me pick a stick from a cup and I got a pink one that google translates to match making. I already have a partner, and picking the omamori made me feel like our relationship is the right one. I'd like the omamori to be meaningful, and I feel like it being promotional and empty makes it feel like a gimmick.

So again, am I allowed to put my own prayer in it? Should I just close it up and carry it around as a sentimental thing?

Thanks for any answers!