Updated: Dec 9, 2022
S Broker is a broker branch of the Sparkasse Group. They even have the same app/online platform. Because of this, you can follow the Sparkasse guide available here. You can even log in to the Sparkasse website with your S Broker credentials.
Please Note; if you have GS2C shares, you can convert GS2C to GME with Sparkasse. You can also convert them to GME and transfer to IBKR at the same time using this guide.