To make sure to NOT promote egg eating, first, make sure your eggshells are clean/dry. I nuke mine in the microwave for 20 seconds after cracking open, then put in a gallon ziploc with the top open. Once it's full, I close the top & rollerpin the heck outta them. THen, I just sprinkle it on top of my dish of oyster shell.
There's no need to microwave, cook, or otherwise prepare the egg shells. I throw mine into a bowl in the counter and let them air dry. When the bowl is full I crush up the shells and mix them in with the feed.
I throw mine in a gallon size bag and into the freezer they go until I have a lot of them, then I just crush them with my hand while still in the bag and throw them out with their treats. No problems with egg eating from my girls.
I have an old pie tin on the furnace that I put the egg shells on to dry. After a couple of days, I drop them in the blender to chop them up... coarse for the chickens, and really fine for quail. Works like a charm.