||KeyTips are highlighted in KeyTips sample. Check
KeyTips appears for all tabs, all controls on the Quick Access Toolbar and the Backstage when the ALT or F10 key is pressed and released.
To define KeyTip for a control use KeyTip.Keys
<Fluent:Button Text="Click Me" KeyTip.Keys="K" />
KeyTips can be attached to any control. Be aware you must attach KeyTip to groups (it uses when group is collapsed)