Did you read the PO PRO documentation, please?
You can change the way taxes are handled from PO Settings:
Now, you can specify whether you want to use taxes or not and how to handle Purchase Price's taxes when you use the "set purchase price" feature from POs.
And, in PO PRO, by default will get the tax rate saved on your supplier when you create a new one and assign the supplier to it, but, later, you can also change this rate manually. So if you want to add a 10% tax to any PO, just add 10 to the tax rate field and click outside so the system will recognize the change and will ask you whether you want to apply the new tax to all the PO items.
Alternatively, you can also edit the tax per PO item line. Just click on the tax figure for any item and a popover will display to edit the value (as fixed amount or percentage).
So, it's very flexible now and you can add any tax you need for any PO 😉