omnibox.setDefaultSuggestion()

Set the default suggestion to appear in the address bar drop-down list when the user starts interacting with your extension.

Once the user has entered your keyword and started interacting with your extension, the default suggestion will always appear first in the drop-down list. Unlike the suggestions you supply in omnibox.onInputChanged, this item can't be selected. Use it to provide the user with some guidance about what to type and what they can expect to see when they do.