Cannot input validation code

Hi,

I'm trying to populate a dropdown field using the result from a previous dropdown field. 
This is the code i found: 

if(event.value == "Item1")
this.getField("Dropdown2").setItems(["Item 1","Item 2","Item 3"]);
else if(event.value == "Item2")
this.getField("Dropdown2").setItems(["Item 4","Item 5","Item 6"]);
else this.getField("Dropdown2").clearItems();

However, when I try to input this code in the "Run custom validation script" box, the code simply disappears when i click OK after editing.. How can I solve this?

Thanks!