Click Type: 'number key'

[CreeperOverLord7]

There is a click type in skript called 'number key'
https://skriptlang.github.io/Skript/classes.html#clicktype

I want to get the actual number pressed (1-9) in an inventory click event, but using the click type expression only gives me that a number key was pressed.
E.g.

on inventory click:
    if click type = number key:
        set {_key} to ???
        #how to get which key was pressed here?

(I have tried searching docs, but to no avail)