How do I add a search box to my store's page-not-found screen?

Turn on the search box in the 404 settings.

1. Go to Settings → Web Settings → 404 Page Settings.
2. Under Choose a design, pick Centered, Big 404, Illustration or Dark.
3. In the edit box below, switch on Show search box.
4. Click Save Changes.

The box searches your shop's products. The same form also lets you change the Heading, Subheading and colours, and set your own logo for this page.