Parasitic Leech not showing up in Eldritch invocations

The new Eldritch Invocation Parasitic Leech from Heliana’s Guide to Monster Hunting part 2 is not showing up as an option in character creation when  you have chosen The Parasite as your patron. 

Wrote Loot Tavern to ask about it 

In the new Heliana’s Guide to Monster Hunting part 2 book, the Parasitic Weapon says that it is a "special attack is a melee spell attack" but when you use the DND Beyond character builder, it is use the Dex modifier instead of your Spell Attack modifier. 

I wanted to find out is the Spell Attack or Dex that should be used for the attack with the Parasitic Weapon?

Thanks in advance for your insight on this.