As far as I'm aware, you just have to wait for the slayer master to give you the special assignments, you don't have to turn anything on unless you've blocked special assignments. If you think you have blocked them, you can toggle them back on by talking to a slayer master.
I've had 2 from The Raptor but they seemed to be given randomly after you've completed at least 10 assignments (from any master). Your chance of getting one increases each time you complete an assignment (or reroll using slayer VIP tickets) but it decreases if you decline or cancel an assignment.
If you look at the RSWiki, it explains the mechanics better than I can here.
24-Apr-2023 03:53:22