You say A>B>everyone.
Give A 9. Give B 5. Everyone else gets zero. Or if you think A and B have no risk of losing, give B 0 so your A v B vote counts more. If you think both candidates are going to lose to another, then vote B 9 so that it stands more of a chance.
In the instant runoff, you only have one option: A>B
As for encouraging a 2nd place candidate, I think I need to do a bit more research. I forget why, but perhaps I was thinking of another voting system.