Assuming that the chat box is on its own track, be sure that track is the top track. Go into Pan/Crop on the event (or clip) on that top track. Click on mask in the lower left corner of the pan/crop screen. The mask controls will become active. Use the Anchor Creation Tool (it looks like a pen tip) to create corners for the chat box. You will see the mask being drawn as you create the anchor points. Finally, click on the first anchor point again to complete the mask. Only the portion of the image that is within the mask can now be seen for the top track. Click on “Position” which is on the lower left of the pan/crop screen just above “mask”. You can now move the image to position the area you selected with the mask where you want it to be. You can also expand and contract using the normal crop/pan size controls (i.e. to expand simply drag a corner of the screen out). As you expand the screen, the masked area will also expand. Contracting the size works the same way.
If the chat box is superimposed on your main video, you will first need to duplicate the track. Then, work on the top track, and follow the above directions. You will, in that case, still have the problem of the covering the chat box on the main video image since your whole goal was to move that part of the screen.
You can find more detailed explanations of how the various controls work in the help screens.