If you just want to apply a set percentage deduction for tax on a licensor, you can do this by clicking in to them from the Accounting > Licensors screen, and changing the 'Apply Tax' toggle to 'Yes'.
You will then need to change the 'Use Default Tax Rate' below that to 'No' (as your default tax rate is currently 0%) then enter the % to deduct from this licensor in the 'Tax Rate (%)' box.
Please note the tax deduction is only shown on statements that are over the payable balance for a period.