Startup Manager on external display

I have a Macbook Air Mid 2013 running Yosemite. My main display is broken due to an accident, but I have been successfully using an external display (thunderbolt to HDMI or thunderbolt to VGA, both works, no problem).

I wanted to make a Bootcamp installation and I have no problems with creating the USB and such.

The problem is, I can never get the Startup Manager screen (after pressing 'option' on chime) to be displayed on the external monitor. My external monitors only pick up the display once I get into the user selection for MacOSX.

Using a few tricks, I have managed to get the monitor to display Recovery Mode(command + R), but never the Startup Disk selection menu. I sort of need Windows at the moment, and this is driving me nuts.

So I need to know, how to force the Mac to show the boot up / startup screen / startup disk selection / apple logo / on an external monitor?

Thank you.