I have tried to achieve this in several ways, none of them work:
<a href="url" target="_blank"/>
<a href="javascript:void(0)" onclick="window.open(url)"/>
<a href="javascript:void(0)" onclick="window.open(url,'_blank')"/>
<a href="javascript:void(0)" onclick="window.open(url,'_newtab')"/>
<a href="javascript:void(0)" onclick="window.open(url,'popup','width=640,height=480')"/>
I read here that this is not possible to control programmatically on Windows Phone 7, however I need a solution for Windows Phone 8.