crafting recipe with custom item

so I want to make a custom recipe with custom item,so what I'm trying to make is a flint axe that requires a plant string
here is the recipe:
pf
s
s is stick
p is plant string
f is flint 
however I do not know how toreference p as plant string
plant string is a custom item I made
sorry if the question is confusing,English is my second language so sorry!