Android Oreo Install Apps from Unknown Sources on Google-free LOS device

I installed LineageOS on my phone without any google related stuff like gapps. So my device is Google-free so to say. The problem I have is: I want to install apps, not from the Play Store, so these apps are from “unknown sources”. In Android Oreo you need to select an app that you grant the permission to install apps from unknown sources which I don’t have. Any ideas?