Windows 8 “Run as different user” isn’t enabled by default for the start page. Here’s how to enable it:
Local Computer Policy > User configuration > Administrative Templates > Start Menu and Taskbar.
Show “Run as different user” command on Start
Ironically, I discovered this while trying to open Group Policy Manager, which is what you use to enable the policies like this one in a domain
I assume this would apply also to Server 2012