Connecting to an external monitor when MacBook screen is broken

HI my daughter sat on my MacBook Air and cracked the LCI. Repair is about 1/2 - 2/3 cost of a new one so I was hoping to connect to an external monitor in the short term. Genius Bar suggested this as an option and said it would be as simple as connecting the monitor.

I have the right connectors (mini display port to VGA Adaptor) and the external monitor (it's HP) shows the fuzzy version of my screen that you get just prior to the login screen but doesn't get any further. The screen on the Mac has three options at the bottom (sleep, restart or shut down) so unless I can get past these and actually login then the external screen won't work.

ive tried restarting with the external screen connected but it defaults to the small portion of the mac screen that works.

is there a way to disable the Mac screen (using keyboard obviously) to make the external screen the default or only screen?

Thanks