There's a couple of issues here: This screen is made up of several sub screens so referencing objects is a little more tricky. You start with the parent screen which the _item widget resides on - the Item Availability Workbench.
As Scott mentions you can then trigger a custom action using something like:
The trick then triggering the correct action on your new tab. This also has to be done inside the parent script and you have the function call actions on the children of that parent screen.