How to create a many-to-many mapping in Entity Framework?

If you want to create many to many relationship with additional data in association table, you have to make the association table as entity. The pure many to many relationship is only in pure table with entity id’s.

In you case it will be:

public class Media // One entity table
{
    public int Id { get; set; }
    public string Name { get; set; }
    public bool Enabled { get; set; }

    public virtual ICollection<ContractMedia> ContractMedias { get; set; }
}

public class Contract // Second entity table
{
    public int Id { get; set; }
    public string Code { get; set }

    public virtual ICollection<ContractMedia> ContractMedias { get; set; }
}

public class ContractMedia // Association table implemented as entity
{
    public int MediaId { get; set; }
    public int ContractId { get; set; }
    public DateTime StartDate { get; set; }
    public DateTime EndDate { get; set; }
    public double Price { get; set; }

    public virtual Media Media { get; set; }
    public virtual Contract Contract { get; set; }
}

And after you created models/entities, you need to define relationships in context:

protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
   modelBuilder.Entity<ContractMedia>()
       .HasKey(c => new { c.MediaId, c.ContractId });

   modelBuilder.Entity<Contract>()
       .HasMany(c => c.ContractMedias)
       .WithRequired()
       .HasForeignKey(c => c.ContractId);

   modelBuilder.Entity<Media>()
       .HasMany(c => c.ContractMedias)
       .WithRequired()
       .HasForeignKey(c => c.MediaId);  
}

Also you can refer to these links:
Many to many mapping with extra fields in Fluent API
Entity Framework CodeFirst many to many relationship with additional information
Create code first, many to many, with additional fields in association table