Unfortunately, % codes like %default, %var and %math don't work in item names directly, you need to use Set Variable: Set Item Name
View attachment 1904
To do it, put a variable in the 1st slot, preferably a LOCAL one (since we will use the item named that specific value only once), in the 2nd slot put your item and in the 3rd slot put the %var(name) text, now you can use the variable instead of the item in stuff like Give Item, Show Inventory, Set Inventory Item and basically anything that accepts items :D