Price / discount overlays are pretty simple to set up, but must be done directly in Facebook Ad Manager:
Navigate to the Ads tab
Hover over your Ad and click Edit
At the ad level:
Edit your ad
Navigate down to Ad Creative
Open the Catalog menu
Now, scroll down to Creative Options > Creative Tools > Add Catalog Info:
Clicking on Add Catalog Info will open the editing menu:
Click on the Info drop-down menu
Select which overlay you wish to utilize
Once satisfied click Save to finalize your selection
You can also edit the color/style and appearance of your overlay from this same menu!
Note that these overlays are based on data passed to Facebook in your Catalog (feed) so certain information may not necessarily be sent to allow for certain overlays.