C# relational database

I have a relational table, where the connection is between users and notes. My problem

1. _result.Entity.Id returns -2147482647

2. I use Email as the key, but when creating more than one note there is an error stating that I already have such Email in the database, I would like to hear a solution to this problem.

3. My task is to match the Id notes in the two tables, but in the communication table id equals (see 1)

[HttpPost]
public async Task<IActionResult> Create(AddToDoViewModel model)
{
    if (ModelState.IsValid)
    {
        ToDo toDo = new ToDo {Name = model.Tittle, Body = model.Body };
        var _result = await db.ToDos.AddAsync(toDo);

        UserToDo userToDo = new UserToDo { ToDoId = _result.Entity.Id, UserEmail = User.Identity.Name};
        var result = await db.UserToDos.AddAsync(userToDo);

        db.SaveChanges();

        return Redirect("/Profile");
    }

    return View(model);
}

public class ToDo
{
    [Key]
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public int Id { get; set; }
    public string Name { get; set; }
    public string Body { get; set; }
}

public class UserToDo
{
    [Key]
    public string UserEmail { get; set; }
    public int ToDoId { get; set; }
}

I will be very grateful for any help