Like everything else that starts with an underscore, _Event is a local variable (type String in your case). In actuality, the name could be anything.
The problem is that you either need to know what the variable is supposed to be, or use the variable itself within the function. I think you could do something like this:
IF
//First function
THEN
CharacterMoveToItem(_Character, _Item, _Event);
GlobalSetEvent("Custom event");
IF
GlobalEventSet("Custom event")
THEN
//stuff