How to specify parent for child model?

Question

ASP.NET

There is one model:

public class User
    {
        [Key]
        public int Id { get; set; }
        public string Email { get; set; }
        public string  FirstName{ get; set; }
        public string LastName { get; set; }
        public string Password { get; set; }
        public int CreatedAt { get; set; }
        public ICollection<Video> Videos { get; set; }
    }

and another:

public class Video
    {
        [Key]
        public int Id { get; set; }
        public string Title { get; set; }
        public string Description { get; set; }
        public string File { get; set; }
        public string Hash { get; set; }
        public int CreatedAt { get; set; }
        public int Views { get; set; }
    }

Communication one-many as you understand. And so, by means of codefirst'a it is all filled in in a DB.
And everything is fine, the User_Id field is automatically created in the Videos table, but damn it, how to fill this field?
That is, when adding a video, how to specify its author? Well, that is, there is a column in the table, but this field is not in the model, so it doesn’t work like this:

VideosRep.Add(new Video()
                {
                    Title = videoTitle,
                    File = FileName,
                    Description = videoDescription,
                    Hash = HashString,
                    CreatedAt = MyHelpers.GetUtcTimeStamp(),
                    User_Id = /// ругается на отсутствие поля у модели оно и понятно
                }
            );

Added a field to the video model
public User User { get; set; }
But when adding a video:

VideosRep.Add(new Video()
{
Title = videoTitle,
File = FileName,
Description = videoDescription,
Hash = HashString,
CreatedAt = MyHelpers.GetUtcTimeStamp(),
User = UsersRep.Users.Where(c => c.Id == userId).First()
}
);

A new user is created in the table, why???