Let's turn on the CIS feature on your contractor's settings. Once you turn on this feature, a new item will be created for use to all customer/vendor associated to CIS. If you're using a Mobile App, I recommend logging in to a browser when following the steps below:
Go to the Sales menu, then the Customers tab.
Select a customer and click the Edit button.
Check the box for Is CIS contractor.
When you use the CIS labour 20% to charge customer, it will automatically reduce 20% to the total invoice.