As Afterpay is an alternative payment provider outside of Shopify, they would have a documentation on how to implement their payment icon onto your Shopify store's footer. I've taken a look at their FAQ page and they have provided instructionshere, under the Shopify Themes heading,on how you can achieve this.
If you're not comfortable with coding or are having issues with integrating this, you can also reach out to Afterpay's team as it looks like they can help with setting up their payment gateway. You'd just need to give them access to your store, as mentioned on theirFAQ page.
Out of curiosity, is there a particular reason you're using Afterpay for your store's alternative payment gateway? If yourstore is eligible, we also have Shopify's very ownShop Pay Instalmentsavailable. Shop Pay Instalments offers customers the option to split their purchase into four equal, interest-free payments too. You can read more about this on ourFAQ page here.
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