m,
You may select the square and Ctrl/Cmd+X+B to move it behind the text.
If you wish to make it a live square/rectangle, which may grow with your live Type, you may:
1) Select the live Type;
2) In the Window>Appearance palette flyout click Add New Fill, then drag it down beneath Characters in the main Appearance palette and select it there;
3) Effect>Convert to Shape>Rectangle, set the size as desired, using Relative (relative will be relative to the Bounding Box of the live Type so you need to calculate the addition to the W and H values in the Transform palette).
That will give you something to start with, and adapting to any changes in the Type.