Prevent a Macbook from sleeping when the lid is closed when power is not connected

The external monitor gets turned off by default when the Macbook monitor lid is closed. Here is how you can override this behavior.  

Open terminal & execute the 

sudo pmset -a disablesleep 1

You can reverse this setting by running the below command in the terminal

sudo pmset -a disablesleep 0

The other regular way for the external monitor to remain ON is as below:

Connect to power.
Connect to external monitor.
Close lid.