The way to add the Instant search widget depends on the theme you use.
1. For Stencil themes
A Stencil theme is installed for new stores by default (Cornerstone is the basic one).
There's nothing you should do from your side. The Searchanise widget is added to your store automatically upon installing the app.
2. For Blueprint (classic) themes
2.1. Go to BigCommerce admin panel -> Dashboard -> Storefront -> MyThemes.
2.2. Click Advanced in the Current theme block and choose the Edit HTML/CSS option.
2.3. You’ll be redirected to the Theme Editor. Open the HTMLHead.html file there and insert the following Searchanise widget code before the </head> tag:
Replace [api_key] with the API Key of the Searchanise engine for your subscription. You can find it in the Searchanise dashboard (BigCommerce admin panel -> Dashboard -> Apps -> Searchanise).
2.4. Click Save.
P.S. In case you experience problems with instant search widget loading, we recommend to try another way of adding the widget. You can see the detailed instructions in this article.