In case the animation effect does not apply to your selected button, you need to enter the proper class of Add-to-cart button in the following box and then click Save.

Step 1. 

Go to your website, and hover on the element you want to be triggered. Right-click and click on Inspect. 

Step 2.

Select the element > Right-click > Copy > Copy selector

Step 3.

Finally, paste your result in the "Custom class for cart button" field in the app: