Orders Form in Northwind Database

I am using a database exactly like Northwind sample database with a little changes. In Orders Form, down, at the end you can find a discount field. In my database I prefer to charge a TAX in place to make a discount and I did in that way that Form automatically apply 5% charge to any order. My question is how to make that the form, also automatically dont apply any TAX in case CompanyName client was (for example)”ANAM”? I dont know, maybe is better to create a system which I can use to decide when to apply (manually) TAX charge or not? (for example a check box or something like that?