CSHARP

ef core add OnModelCreating foreign key

public override void OnModelCreating(...)
{
    modelBuilder.Entity<Position>()
         // a position has zero or more Employees via property Employees
         .HasMany(position => position.Employees)
         // every Employee has exactly one Position
         .WithRequired(employee => employee.Position)
         // using foreign key Position_id
         .HasForeignKey(employee => employee.Position_Id);
}