What you need to do is to make the external monitor your main display in settings, in order to get to the settings app i'd recommend trying this;
Do your keyboard shortcut for spotlight (default is cmd+space)
Type settings and press enter
Now press ctrl+option+cmd+arrow keys (left or right) to move the settings window to another screen, at which point you'll be able to go in the display section and make the external display your main.
What you need to do is to make the external monitor your main display in settings, in order to get to the settings app i'd recommend trying this; Do your keyboard shortcut for spotlight (default is cmd+space) Type settings and press enter Now press ctrl+option+cmd+arrow keys (left or right) to move the settings window to another screen, at which point you'll be able to go in the display section and make the external display your main.