Since the MouseLeftButtonEvent is the built-in silverlight mouse event, you can not raise outside of the framework element you're targeting. You may refer to this forum for more information. What I can suggest you is to use automation peers instead. So you can get the respective automation peer for the RadRibbonTab and force Selection through the peer which should behave exactly as it is clicked.
Please let us know if this is what you need.
All the best,
the Telerik team
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