Finally one that I can answer. The step you are probably missing is having both your quad and the text box inside of a layer object. Once that is done, on your scene font that you are using, select it, go to Edit Material. Then towards the bottom of that new window select subtractive in the blending mode.
If you want to get super crazy, you can use visual logic to set the height and width of the quad based off of the size of your text box. That way the quad will scale with the text (for the most part).
Visual Logic. Boundingbox.height of text box goes to the quad height. Boundingbox.width of the text object goes into the width of the quad. I went ahead and added an add block and added a value to the width so that way I could create more space on the outside, depending on how much was needed.
#XPression