You could try this:
API
authenticate: function(){
return $resource(api+"auth/:usertype",{},post:{method:"POST"});
}
Note: :usertype in URL means that the value of usertype property which you passed into postData will replace the part of URL
Controller
var postData = {email:scope.main.email,password:scope.main.password};
API.authenticate().post({usertype:'player'},postData,function(response){
console.log(response);
});
Or you could fetch response like this:
var response = API.authenticate().post({usertype:'player'},postData);
Hope this is helpful.