Junction Table in ASP .NET code first approach - asp.net

Question

I am creating a junction table between Identity User and a Game table. This table is called UserGame and has two foreign keys (UserID, GameID) and one additional field to store the score.
public class UserGame
{
[Key]
public string ApplicationUserID { get; set; }
public virtual ApplicationUser ApplicationUser { get; set; }
public int GameID { get; set; }
public virtual Game Game { get; set; }
public int Score { get; set; }
}
My confusion lies with creating a new UserGame record. How would I go about doing this and is my approach correct?
Update (This worked):
[HttpPost]
public ActionResult SubmitScore(int gameID, int score)
{
var playerRecord = new UserGame();
playerRecord.ApplicationUserID = User.Identity.GetUserId();
playerRecord.GameID = gameID;
playerRecord.Score = score;
return Json(new { success = true });
}

Answer 1

Both ApplicationUserId and GameId must have a [Key, Column(Order = 0)] attribute. Just set the first to Order 0 and the other to 1.
public class UserGame
{
[Key, Column(Order = 0)]
public string ApplicationUserID { get; set; }
public virtual ApplicationUser ApplicationUser { get; set; }
[Key, Colum(Order = 1)]
public int GameID { get; set; }
public virtual Game Game { get; set; }
public int Score { get; set; }
}
Then you have the choice to add new record, go through nav property from Game or ApplicationUser or directly with your UserGame class.

Answer 2

Example of configuring many-to-many relationship in entity framework
Examle of inserting related objects
A couple of tips:
I wouldn't declare a primary key in the junction entity as junctions are usually defined by composite keys.
Keep in mind that dbcontext.SaveChanges() will look for child entities and save those as well.