I wish I'd taken a pic of the parts but I didn't think of it. I used a 2" PVC elbow (I think 90 degrees), a short, 4" - 6", piece of 2" PVC and secured it to the top of the elbow, a 2" pipe connector (thick black rubber with hose clamps, you can barely see it in the picture) securing one end to the short piece of PVC. I drilled a hole on the side of the plastic planter near the bottom. I just used some cardboard to support under the elbow and used hot glue to secure the elbow to the planter. Once the elbow was secured in place I filled the bottom of the planter with cement. I used the small bucket of patching cement from Home Depot. In retrospect I should have gotten the larger bucket. I can now secure the light post to the top of the elbow by inserting it into the connector and tightening the hose clamp then I feed the electric cord down through the light post and out the back of the planter.
I did have one boo-boo. I decided to leave the lamp post connected for the ride home and naturally it tipped over. It knocked the elbow loose. Since the small bucket of cement wasn't enough to cover the elbow to the vertical portion of theelbow, the cement created a tunnel and now the elbow will rock back and forth. Make sense? What I'm going to do is drill a couple of screws partway into the top of the horizontal elbow piece and fill it in with more cement. The cement will settle around the screws and the pipe shouldn't move anymore.
That's the way I did it. I'm sure there are several diffrent ways. Maybe you'll think of something better. If you do though you have to post it.