You can use the WebBrowser Object
to open any new page at any time you like using normal events
this way you could for example..when the game is completed
have a little popup or new layout that asks the player nicely to goto the playstore
and then either:-
- open the link for them in a new browser window
or
- just give them a link to click on themselves
not every player likes to be hassled by stuff like this..so do it gently and you should have no dramas