You cannot -- you either need to fly the screen blind, or replace it.  
Hi Guys,   Broken LCD in my laptop is not allowing me to reinstall Windows, because at the first stage, it does not recognize the external monitor on HDMI.    Wythout it I can't follow steps to reinstall.  Any guide how can I force this Inspiron5559 to use external as primary screen before Windows starts? Best,  Bartosz
Windows reinstall on laptop with broken LCD.