You can access it here when you have the mtext editor open:
Then select Background Mask...
You get a window like this:
Be sure to check the Use drawing background color box and the border offset factor to something just slightly larger than the text. I thought 1.2 was pretty good.
You can use this to get our text to show up better when it is on top of architectural items that make it hard to read the text. I don't like to place text over lines or architecture but if I have to, I like to use this method. I don’t need to use it all the time but it would be useful in some situations.
Uncheck the "Use drawing background color" to use a different background to highlight the text box if you want.