I've had one of these tablets for about a week now, and the share panel IS annoying. It's activated by swiping down into the touchpad area from outside it.
Unfortunately, it appears that THIS is the only way to capture screenshots besides using the snipping tool - Windows icon and vol-down doesn't work on mine either. I succesfully managed to capture a screenshot and 'Share' it to OneNote, although on mine I could have chosen 3D Builder, Cortana Reminders, Facebook, Fresh Paint, Mail, FB Messenger or Twitter. It looks like only UWP apps are supported, AND they have to be programmed to accept Windows 10 Sharing API (I can't share to Paint, Word or Photoshop for example).
I'll keep digging around in the settings and see if there's a way to reactivate the hardware screen capture, but it's likely to have been limited by the manufacturer.