You need to add the script code found in the App settings section here to Script Manager section of your Bigcommerce store:
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Depending on Theme you are using in your Bigcommerce store please click the radio button like on the screenshot above and then copy the script to Script Manager:
Click Storefront on the left side of your Bigcommerce store admin panel.
Then Script Manager.
And then click Create a Script.
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On the next page you need to name the script (for your internal use) e.g. Unlimited Upsell
Description can be skipped.
Choose Footer for Location on page.
Choose All Pages for the option Select pages where script will be added.
Choose Essential for Script category.
Choose Script and insert the script from Settings section of our app:
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Click Save.