Pick the center point and then pick the origin and make them coincident. You might also have to constrain one of the lines to be vertical/horizontal.
Your question is so basic it is evident you are a newcomer. This forum is a great place to learn, but there are things that will help you to get more out of SWX and more out this forum.
First and foremost, go through the built-in SWX tutorials. They are extremely well done and you will dramatically shorten your learning curve.
Secondly, go through this post: How do I do this SOLIDWORKS Forum thing? It includes how to attach a file to your post. When you attach a file you give us a better idea of what you have tried and you get better help.
Welcome to the forum! Now do your homework. :-)
Since this is a 3d sketch, even if the center of the rectangle is coincident with the origin it won't be fully defined since it's not restricted to a plane and can revolve in any direction. If this is all that the 3d sketch will consist of there's no reason to make your life harder by creating it as a 3d sketch. They're much more difficult to control and define than 2d sketches, and should only be used when necessary.
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