Since it's indoors you can hang hooks from the rafters, tie a string or wire to the stem at whatever you feel is the right hight (somewhere below your
lights I presume) and let the rest just hang down from that. I read somewhere that commercial growers use this technique but the allow two main branches to survive and train them along a string in sort of a teepee formation.
Make sure you avoid damage to the stem by using a plant clip or a "Y" connection of some kind. The idea is that the plant will become quite heavy and the point where it is tied off will otherwise get considerable stress.