As Sezzle is an alternative payment provider outside of Shopify, they would have a documentation on how to add their widget onto your product pages. I've taken a look on their website, and they have provided instructions onhow to add the Sezzle widget here.
If you're still having trouble integrating the Sezzle widget onto your storefront, you canreach out to Sezzle's teamto see if they can help you implement this on your store.
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