(http://wpmegamenu.com/help/)

[ubermenu-search]

- Activate Content Overrides in
UberMenu > Descriptions, Shortcodes, Widgets > Allow Content Overrides

- Add a Custom menu item to your menu

- Set the Navigation Label to 'Search' and the URL to '#'

- Check Disable Text

- Check Disable Link

- Check Align Menu Item to Right Edget (optional)

- Add the shortcode
[ubermenu-search]
to the Content Override textarea

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Follow the link to read about customization (PHP and CSS).