Prerequisites
- Custom search engine configuration must be completed

Edit config.toml
To add it to the menu bar, add the following:
[[Menus.main]]
Name = "search"
URL = "/search/"
Add index.html under content/search
---
title: "Search"
---
<form action="https://cse.google.com/cse">
<div class="searchBox">
<input type="hidden" name="cx" value="1ef1df26a465b967e" />
<input type="hidden" name="ie" value="UTF-8" />
<input type="search" name="q" placeholder="Google Custom Search" size="30" autocomplete="off" />
<input type="submit" value="Search" />
</div>
</form>
Result
The menu bar and search box are now ready.

The URL format is as follows, which allows searching from a command-line launcher:
https://cse.google.com/cse?cx=<search-engine-id>=UTF-8&q=<search-string>