@schratze i struggled with this too, you need to get the boot.img from the lineageos image.

https://wiki.lineageos.org/extracting_blobs_from_zips#extracting-proprietary-blobs-from-payload-based-otas

there you'll find the boot.img, you have to transfer that to the phone, use the Magisk manager app to modify it, then flash the modified boot img to your device https://topjohnwu.github.io/Magisk/install.html#patching-images