Multi-year advance flat rate in the tax return when selling

I hold some accumulating ETFs with a foreign broker and have decided to sell them. Since I have to calculate the advance flat rate myself every year, I will, as usual, have to declare the advance flat rate for 2025 in the tax return for 2026 next year. This means that I have to declare the advance flat rate for 2025 and the profit taxes for 2026 in the same tax return. How does this work? Is there a special field in the tax return where I can enter the tax return?