Nope you can't make it Mandatory using existing code. There are hacks to do it but its not officially supported.
While you can't make Description or Reason for Request mandatory. You could capture this information with a custom property.
Create a property in properties dictionary (eg. MandartoryDescription).
Set the control type to Notes and Required to yes.
Add this property to your blueprint or a build profile with prompt user checked.
Now you will have a field just like description but it is required.
You will still have the Description and Reason field but you will now provide a field to capture the same information that is mandatory.
Will this custom property "Notes" populate the Notes in the annotations field for vSphere deployments, just like the description on the request form does?