How to add extra amount to individual tax total?

I have two tax rules in my online shop.

1. Tax 1

2. Tax 2

The tax calculation is based on the subtotal. But now I want to add custom amount with the above tax .

i.e., Tax 1 = Tax 1 + custom amount Tax 2 = Tax 2 + custom amount.

Where I need to change the logic in order to achieve this ?

Any help is really appreciated.

Thank you