Just try to install the app and it should ask you if you want to give permission to download from unknown source.I'm testing a new app that is in development. How do I install apk files/apps from unknowns sources on my Note 8? I used to be able to do this in Android 7 but they took this out in Android 8.
It's still there.
You have to enable developer mode.
Samsung Galaxy Note 8 - How to enable developer options - TheCellGuide