EF Core: How to add the relationship to shadow property?

I have two classes:

public class DbLibrary
{
    public Guid Id { get; set; }
    public string Name { get; set; }
    public List<DbDepartment> Departments { get; set; } = new List<DbDepartment>();
}

public class DbDepartment
{
    public Guid Id { get; set; }
    public string Name { get; set; }
}

In this model I need DbDepartment hasn't the property contained the link to DbLibrary. But I need the cascade delete on the database side. For this purpose I add the shadow property to DbDepartment. It is the foreign key. How to relate the primary key DbLibrary class with the shadow property?

This is my attempt:

protected override void OnModelCreating(ModelBuilder builder)
{
    // Create the shadow property
    var id = builder.Entity<DbDepartment>().Property<Guid>("LibraryId");

    // Create the relationship for cascade delete
    builder.Entity<DbLibrary>().HasMany(n => n.Departments)
        .WithOne(m => id /* it is wrong */).OnDelete(DeleteBehavior.Cascade);
}