how to (INSERT) data in this table in entity framework code first?

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using EF 6
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I have 3 table :

1) Users

public class Users 
{
    [Key,Required, MaxLength(50)]
    public string UserName { get; set; }

    [Required, MaxLength(128)]
    public string Email { get; set; }

    [Required, MaxLength(128)]
    public string Password { get; set; }

}

2) Roles

public class Roles 
{
    [Key,Required, MaxLength(50)]
    public string RoleName { get; set; }

    [MaxLength(50)]
    public string ApplicationName { get; set; }
}

3) UsersInRoles //My Problem.

public class UsersInRoles 
{
    [Key]
    public int id { get; set; }

    [ForeignKey("Users"), Required]
    public string FKUserName { get; set; }
    public virtual Users Users { get; set; }

    [ForeignKey("Roles"), Required]
    public string FKRoleName { get; set; }
    public virtual Roles Roles { get; set; }
}

Now my question is how to insert record in UsersInRoles table ???

i use following code but don't insert any data in table!
even don't show any errors!

    List<UsersInRoles> BUsersInRoles = new List<UsersInRoles>
    {
        new UsersInRoles { FKUserName="User1", FKRoleName="Role1"},
        new UsersInRoles { FKUserName="User2", FKRoleName="Role2"}
    };

    foreach (UsersInRoles BIR in BUsersInRoles)
    {
        db.UsersInRoles.Add(BIR);
    }

    db.SaveChanges();

Thanks.

Update: i have found a error :

The INSERT statement conflicted with the FOREIGN KEY constraint "FK_dbo.UsersInRoles_dbo.Roles_FKRoleName". The conflict occurred in database "AriyaRad", table "dbo.Roles", column 'RoleName'. The statement has been terminated.