Input Tax in PO as a fixed amount not percentage

Dear Experts

I had an issue that we need to apply a tax for the items in PO but I need this tax to be a certain value (not a percentage from the item value )

So I tried to maintain the tax condition in the PO item condition  manually but the problem is that the system couldn't recognize that this is a tax on the item and in the invoice the system couldn't determine the tax

So how could I add a tax to a PO line as an amount not a percentage in PO and the system recognize it is a tax

Best Regards