Select the Developer tab> Legacy Forms> Form Field Shading
After you have selected the gray area disappears and you can use this text.
If the Developer is not found, it is because Word 2007 does not show this tab in the default mode and to display it you need to click the Office Button, select Word Options.
In the Popular section on the right, check the Show Developer tab in the Ribbon, and then click OK.
So everything has been solved in a very simple way. Good luck!