Setting a fixed Tax Rate on a Pay Component Sub Type (PCST)
If you are creating a new Pay Component Sub Type that requires its own tax rate then below is an example of the fields that you need to set.  if you scroll to the bottom of the page you will see the System Set Fields section. Complete as outlined.  If the PCST already exists then you can edit it and also set these fields.
To access your Pay Component Sub Type List navigate to Payroll Administration > Payroll Setup > Pay Component Sub Type
Also make sure the "Taxable" box on the Sub Type in the first section is ticked.