I can't speak specifically to Folio, but I recently received an offer of rep for my memoir, and one of the first things my agent asked me was, "Would you be willing to write up a proposal?" Fortunately, I'd already created one after being asked for a proposal by another agent who was interested in my manuscript. Bottom line: I would strongly recommend creating a proposal as soon as possible, but I wouldn't let it stop you from querying. If an agent loves your manuscript, they'll ask for a proposal. They certainly aren't going to turn away a manuscript that they love if you don't have a proposal ready.