Before you go to launch, bring the bar towards the chicken loop, (maybe a third of the way down to begin with) and keep it in place with the adjustable bar-stop. This will put tension on the rear lines and help hold the kite up.
Position the kite at the edge of the window for self-launch and it should now stay vertical - it may bob up and down slightly
As you trot back towards the bar, keep your hand on the upper rear grey line (you'll have to jump high to get it!) to aid control of the kite position if the additional bar tension on the lines is not quite enough.
As you get to know the kite, you'll learn where on the plastic tube your bar and adjustable stopper needs to be.
On launch, be aware that the bar is already tensioned and that the adjustable bar-stop, may now not be so slidable. This may affect the amount you can depower. It depends how tight you squeezed the locking mechanism. I err on the side of caution and squeeze it up "medium"
Hope this helps