Can't use array function in array pop action

In array pop i can't write an array function unless i put it inside a variable first.

For example if i want to write inside the field Position in array pop action this function: %array(#?index) it won't allow me to do so because the keyboard is only keypads (numbers).

My suggestion if possible is to change the keyboard or to have an option inside the variables list to create a new one because right now i can only choose a variable from the list.