Manaflow Band is what makes Xerath Support possible. His [Q] and [W] mana costs are extremely high with without Manaflow you're going to struggle poking and harassing in lane.
Try to use [Q] when the Manaflow effect is up so that you get the most out of the mana refund.
TIPS: Always try to auto a minion when your passive is off of cooldown, to further sustain your poking pressure.