How to add a new tax to the order

[Levi]

Hi,

I want to add a new tax, to the orders. I created a form to the order page, and i save the new tax value and the order_id to a DB table.

My question is, how can I add this tax value to the order value? Is there a solution to this?
 What files should I make the modifications?

I just need one function same as PrestaShop "discount", where I can increment the order total value.

Ps 1.6

Thank you,

Levi

Edited September 21, 2018 by Levi (see edit history)