Open the terminal application on your computer with the SDK Platform Tools installed and enter adb devices

This should trigger a popup on your device. Make sure to have the checkmark ticked and click Allow.

I haven’t added the platform-tools-folder to PATH. That’s why I need to open my terminal window in the platform-tools-folder and enter .\ as a prefix to my commands.

You can run the command again to check if it worked. It should now say device instead of unauthorized.