You can use Click on an exact area setting to trigger an upsell offer right before your customer add an item to their cart.
To activate this function, on Triggers section > Add new trigger > Choose the On click to a specific area
An editor popup will show up, here you can give the detailed element that when being clicked will trigger the banner.
You can either inspect the code line on your webpage to find the right element/ Or you can simply use the "Find element" function for easier selecting.
A new popup window will appear, click the Start Selecting button on the toolbar, and for this example, highlight the Add to cart button on your product page > Save. The element box should be filled with the exact element class of the Add to cart button.
Now the last thing you need to do is designing a nice banner for your upsell product, that will show up right when customers adding a product to cart.
You may also want to learn about the other Triggers of when to show your Banners here.
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