Come with the business question, not a channel wishlist — what decision are you actually trying to make, and what happens if it goes unanswered for another quarter. Rough numbers help: current spend, conversion rates, team size, and what's already been tried.
It also helps to know your timeline and whether you're looking for a defined Project, an embedded Retainer, or a short Sprint, though Kando can help narrow that down on the call itself. Most companies get more value out of one focused call than a written brief.