Hold the bar with one hand and put the other on the platform support. With both arms straight, lift the bar, make a little jump and at the top of your jump, take the bar with the other hand before swinging down.
You can find out more about this trick in my books:
IMAGE CREDIT: 1st: Diana 2nd: Thanks to Emerald City Trapeze Arts. Flyer is Nathan. Thanks to David Treadwell for the photo.