Getting the SymbolInstance for an element inside of a symbol

Symbols
16

If I understand right.

In the Symbols Custom behaviours, set a behaviour to fire the Javascript.
Name the behaviour the same name or id as the Symbol.

Set the Symbols timeline Action to fire the Custom behaviour.

Then in the Javascript you can use

Something like:

var theName  = event.customBehaviorName
var thisSymbol = hypeDocument.getSymbolInstancesByName(theName);

( I have used the name instead of the ID here )