In that case you have to go to the tax office and file out some documents and then you pay the taxes for the investments separately.

There is a single income tax rate, 10%, so it doesn't matter how much you earn. If your investments make more than the annual minimum wage, you also have to pay the health insurance tax and "social security" tax.

I assume it’s done the same way as any other withheld tax. The only difference is negotiating salary net of tax rather than ex-tax.