How to manage (create, define, ..) variables?

I want to create new items based on automation (any time period) and ‘Previous Month’ must be part of the newly created Item Name.
After pressing ‘$’ in the Item Name field, a limited variable list is displayed. The variable ‘Current Month’ is in the variable list, but ‘Previous Month’ is not.
How can I create the variable ‘Previous Month’ and make this variable part of the variable list?