An explanation for subprojects can be found here.
Per your example, you should define the routes in your subproject as
GET /admin/rate controllers.subproject1.Application.rater(id: Int ?= 0)
Then in the view you would call the route as follows (assuming you pass an id variable to the template):
<a href='@controllers.subproject1.routes.Application.rater(id)'>asdf</a>