Hello,
Setting that property to true is the right thing to do. It's effectively what clicking the app button does. If you can't figure it out, then make a new simple sample project (as minimal as possible) that shows it happening and email that to our support address. Reference this post in your email and rename the .zip file extension so it doesn't get spam blocked. We'll take a look at that.
If you look at our BackstageIntro QuickStart codebehind, you can see where we set the initial tab in response to an app menu open notification event.