I'm not sure; I've only grown common willow and just allowed it to grow naturally so that side shoots branch out from the main shoot. It's probably a good idea to keep the shoot upright. Otherwise, the growth may end up lopsided. Keep it upright for this year and see what happens next year. Side shoots will then grow, and the main shoot will be thicker and start to become self-supporting. Make sure any ties you use don't cut into the delicate stems. A good tip is to thread ties through short sections of waste garden hose or similar to make them wider and prevent them cutting or abrading stems.