If you do a search on Fabletics from the Home screen in the app, you will find the page for this store. You then need to press the Shop button to get credit.
If you use the Browser Extension, it should automatically pop up when you go to their website. If it doesn't, you can go to ibotta.com, sign in and then select Fabletics.com from the list of retailers and then activate the offer.
Please note that it says you must complete your shopping within 45 minutes.