From Leopard Gecko Wiki
Calcium is a suppliment that leopard geckos need for survival. There are two ways of dosing your gecko with calcium. Supplying a gecko with calcium also prevents MBD.
This is by far the best way to give a leopard gecko calcium, but it requires the best work. Dusting insects can also be messy. Reffer to Supplements for more information.
A calcium dish is a shallow dish or bottle cap filled with calcium powder. The lizard may lick the calcium out of the dish, thus dosing itself.