Go to the "Settings" option in the main menu.
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Under "Website", click on "Advanced".
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In the "Code Injection" section, you will see "Header" and "Footer" options.
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To add the custom Javascript snippet to all pages, you need to add it to the "Footer" section.
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Paste your Amped Code Snipept into the "Footer" section:

Your Amped Code Snippet can be found in your account in the top right hand menu under Integrations > Manual Code Snippet Install and it will look something like this

​<script defer src='<https://app.amped.io/snippet/account_ACCOUNTID>'

</script>
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Save the changes and you're all set!