Out-of-the-box Tabular doesn't let you set the DefaultMember on dimensions or measures. This limitation is probably purposeful since DAX queries (i.e. Power View) would ignore this default member. However you can edit the XMLA and set the default member by adding the following to the .bim file at the end of the Commands tag:
Update Dimension [Date].[Calendar],
Default_Member = [Date].[Calendar].[Year].& ;
However, we should carefully consider before implementing this feature since there's no parity with Power View.