I need to add a tax that it's defined by the vendor, not calculated from the sales price or whatsoever, but from their own buy price and some other stuff (like the volume when they bought, etc) that is not know to calculate it from.

NOTE: This is the gas/diesel tax in Chile, where it's calculated from the buy price/volume. Example:

They buy diesel at 100 CLP the volume was 10 so the tax is (just an example) 20 Then they sell it at 200CLP plus the tax plus the sales tax (19%).

Can i add a tax that asks me for the amount in each order?

EDIT: I can add custom taxes in the tree below. Sorry for this question