Is it this one?
If it is, adding a thumb safety is a little more work. Here's why...
This is the rear of the gun pictured above:
Here is the rear of a newer design:
The red arrow is pointing to a little square. That square is the plug S&W installs when they omit the thumb safety. If you had this design, you could just pull out the square plugs on either side and put in the thumb safety and plunger.
However, if you have the design I first pictured, you don't have the little square plugs. You can still install a thumb safety, you'll just have to cut the frame to make the relief for the thumb safety. Do you want to do that?
Again, my recommendation is to use it as is. Hundreds of thousands of M&P owners have used their non thumb safety guns without incident. It's really not needed.