This is possible by creating a custom tax rate.
To create a custom tax rate
Go to the Tax Rates page
Click Create New Tax Rate
Select external tax as the price display method to create
Steps to link a custom tax rate to a service
This can be accomplished by linking the external tax rate created above in the service settings.
Service List Access
Select the service for which you want to change the tax rate
Select the tax rate for the external tax created in Pricing