Add the widget to your theme
In your Shopify admin, go to Online Store > Themes.
From the theme's Actions menu, select Edit code.
Open theme.liquid under Layout.
Find the closing
</head>tag, just below{{ content_for_header }}.Paste your Userback widget code right above it.
Save and publish the theme.
Some themes use a variant of these tags, such as [/header] or {/head}, so if you don't see an exact match, look for the closest equivalent in your theme's markup.
Add the widget to checkout
Go to Settings > Checkout.
Scroll to the Order processing section.
Paste your Userback widget code there.
Save, then preview your checkout page to confirm the widget appears.