ᐅ Ikea Ringskär single-handle faucet clogged with limescale / aerator missing

I have the Ringskär single-lever mixer tap in my kitchen. The aerator is now clogged with limescale. However, I am unable to unscrew the aerator like with other faucets. Can someone help me? Is there perhaps a special tool for this?

Thank you for the great tip! I went to Ikea and ordered a replacement key. It was sent to me at no cost.
Best regards