Halp.cs

using System;
using System.Collections.Generic;
using System.Linq;
using Microsoft.Data.Entity;
using Microsoft.Data.Entity.Metadata;

namespace ConsoleApp1
{
    public class Program
    {
        public void Main(string[] args)
        {
            using (var context = new ApplicationContext())
            {
                //context.Users.Add(new User {UserName = "Admin"});
                //context.SaveChanges();

                var user = context.Users.Single(u => u.UserName == "Admin");
                user.Tweets.Add(new Tweet
                {
                    Text = "Hello, World.",
                    User = user
                });
                context.SaveChanges();

            }
        }
    }

    public class ApplicationContext : DbContext
    {
        public DbSet<User> Users { get; set; }

        protected override void OnConfiguring(DbContextOptions options)
        { 
            options.UseSqlServer(@"Server=(localdb)\mssqllocaldb;Database=ConsoleApp1;Trusted_Connection=True;MultipleActiveResultSets=true");
        }

        protected override void OnModelCreating(ModelBuilder builder)
        {
            builder.Entity<User>().OneToMany(t => t.Tweets).ForeignKey(t => t.UserId);
        }
    }

    public class Tweet
    {
        public int Id { get; set; }
        public int UserId { get; set; }
        public User User { get; set; }
        public string Text { get; set; }
    }

    public class User
    {
        public int Id { get; set; }
        public string UserName { get; set; }
        public virtual ICollection<Tweet> Tweets { get; set; } = new List<Tweet>();
    }
}