For investment income of up to € 1,000 or up to € 2,000 per year for spouses/partners, you can avoid the withholding of capital gains tax (final withholding tax) by issuing an exemption order to your bank. You can obtain the form for the exemption order from your bank.
If your investment income exceeds € 1,000 or € 2,000 per year, the bank may only credit you with investment income without withholding tax if you have submitted a non-assessment certificate. You can apply for a non-assessment certificate from the tax office if your income is so low that you do not have to pay income tax on it.