Apple Magic Mouse does not have a middle click facility so you have to use a 3rd party app to enable it. MagicPrefs was my choice but it is now no longer maintained or supported since macOS 10.13.2. due to some Apple technical issues and changes to their API's.

Up until macOS 10.14 I was able to keep it going beautifully but with macOS 10.14.1 it stopped working. But have now managed to get it working:

 

1. If MagicPrefs is not in Menu Bar: System Preferences > MagicPrefs > Show Icon > close System Preferences
2. Click MagicPrefs Menu Bar icon and select Quit MagicPrefs
3. System Preferences > Security & Privacy > Privacy >
4. Click the lock bottom left to unlock > Select Accessibility
5. Enable "MagicPrefs.app"
6. System Preferences > MagicPrefs (click once and icon will reapear in Menu Bar) MagicPrefs is active again.
7. If Middle Click is not working ensure you have: System Preferences > MagicPrefs > Clicks & Taps tab > select One Finger Middle Axis Click > Middle Click

You can download MagicPrefs from here it is free.

Hope this is of assistance to those who like to have Middle Click on there Apple Magic Mouse.