When asking your horse to lunge out on the circle, your first cue should be to turn your body language in the direction you want your horse to move, directing and pointing your hand in that same direction and raising your energy and intention. If the horse ignores or doesn't pick up on that pressure you can add a little driving pressure towards their hip until they start to move in the direction you are pointing.
At that moment you should release all pressure and reward the horse, before evolving the question until the horse can comfortably walk out on the circle and maintain direction and speed on their own.
If the horse tries to come in, you can open your rein and help guide them back out on the circle or add a little driving pressure. Once they are on the circle make sure you body language stays positioned to in front of the horses nose.