How Do I make a Project Entity User Field become Mandatory when a certain Defect Severity was set?

Hello guys.  Need help..

I have already created a Project Entity user field in ALM's defect module.

My question is, how do I make it become Mandatory field (in RED) when a user sets a Defect's severity to a certain value, for example "Critical".

I am using HP ALM version 12.01.

Thanks in advance!

Drew