c# - Entity Framework Core Cascade Delete on One-To-One Relationship

I have a one-to-one relationship between Service and Lexikon while the Service has a foreign key to Lexikon. My Service doesn't need a Lexikon so I made the LexikonID nullable. But my Lexikon can only exist if it is related to a Service so I made the navigation property [required]. I'm using Code First so EF builds my Database.

 public class Service
 {

    public int ServiceID { get; set; }

    //Foreignkey
    public int? LexikonID { get; set; }

    //Navigation Properties

    public Lexikon Lexikon { get; set; }
 }

 public class Lexikon
 {
    public int LexikonID { get; set; }

    //Navigation Properties
    [Required]
    public SoftwareService SoftwareService { get; set; }

 }

I'm trying to delete the data automatically in table Lexikon if i delete my Service but unfortunately I'm not sure how to do this since I'm new to EF.

I tried this code in dbContext:

       modelBuilder.Entity<SoftwareService>()
                .HasOne(l => l.Lexikon)
                .WithOne(s => s.SoftwareService)
                .HasForeignKey<SoftwareService>(l => l.LexikonID)
                .OnDelete(DeleteBehavior.Cascade);

but when I delete my Service the data in table Lexikon will still be in it. On the other hand when I delete the lexikon it will automatically delete the Service which is nice but I also want it to be happen to my Lexikon when I delete my Service.

Unfortunately I'm not familiar with the OnDelete-Method so I hope someone can help me.