Hi,
If you have set the discount for each product, the total amount in the order is already discounted price.
The discount for each product can be set in the record of the Order Line.

And add the order lines into the order, the amount of the order will be updated automatically.

The Discount in Order form is the discount for the whole order and has nothing to do with the discount for products.
It will also be calculated in the invoice automatically.
If this helped you, I'd appreciate it if you'd mark this as a Verified Answer, which may in turn help others as well.
Best Regards,
Nya