Small update: I found out that the leverage is applied automatically and one cannot change this manually on IB.
This is what I misunderstood how trading on margin works. I was used to being able to manually set the leverage as it is possible with the CFD broker etoro.
So i didn't know that other brokers are handling this differently.
E.g. I have another broker account (tastytrade) there margin trading gets suspended, if the cash falls under a minimum margin amount (2500 in my case). In that case trading is effectively done 'by cash'.
So little note for myself: Always check the broker
Probably this info is well known for most of you. But maybe it helps others that are new to active trading like my humble self.