Step 1: Log in to your WisePops account.
Step 2: Open the websites management page (direct link).
Step 3: Next to the website's URL, click on the ... symbol and select "Tracking code".
Copy the code:
Find the header.php file in the following OpenCart folder: Catalog > View > Theme > Template > Common
Open the file and paste your WisePops tracking code before the closing</header> tag.