I would make a tea with the cinnamon and some hot water. Let it steep for ~5 minutes, then strain through a coffee filter. Mix some with a measured amount of beer and adjust from there to get the balance you want. Then scale up and mix the tea into the bottling bucket after you bottle the first “clean” half.
This method allows you to tweak the flavor so you don’t over/under do it, and it gets the little bits of cinnamon out of the bottle. You could also try a longer extraction with vodka at room temp, but I’ve been happy with the spice tea method. Good luck.