In the case of a tax that withholds 15% of the 19% VAT:

We have a product that costs $100.00, we apply the 19% VAT, which would be $19.00.

From those $19.00, we have to withhold 15% (19 x -0.15) = -2.85 (it has a negative sign because it is the amount we are going to withhold).

To configure this withholding in the tax, we need to do the following calculation:

If $2.85 is 15% of $19, what is the equivalent percentage for $119?

Calculation: ($100 x -$2.85) / $119.00 = -2.3950%

In other words, when we apply the 15% withholding on the 19% VAT, we are actually withholding -2.3950% of the product price.

The configuration in Odoo would be as follows (you just need to verify the withholding accounts).

And the invoice would look like this: