Automatic discount when people fill in their tax number

Hi,

There is no option for that for the discounts.

However, you can have a negative tax rate and a tax rule with that tax rate limited to the "company with VAT number" client type.
That way, the tax rate will apply after a customer enters a VAT number.
And if you already have tax rates for normal taxes, what you can do is activate the "cumulative tax" setting of your tax rates. That way, both your normal tax rates and that "discount" tax rate will be able to be applied at the same time.
So like that, you should be able to do what you want.