How To Add Custom Button in Sales invoice item ( Child Table )

I want to add the Custom Button in the Sales Invoice Item which is the child table of Sales Invoice.
Also, I want to perform some Custom Script on that button to try Client Events. As shown in this pic when I am adding a custom button in Sales Invoice Item , the button is not showing there.
Please help me with this issue.

Thanks in advance!

Check below :

@sanjay thanks for helping me