WordPress doesn’t make it easy to add a search form in the navigation menu. It’s a popular request, though, and having access to the search in the site header is typical.
Relevanssi itself doesn’t have opinions on the user interface. Some themes come with a built-in search feature in the header, like Twenty Twenty. That’s a good reference if you want to build a nice search form in the header.
If your theme does not include a built-in search form and you’re not interested in building one yourself, one of the best plugin options is the SearchWP Modal Search Form. As the name says, the plugin is from the SearchWP developer Jon Christopher, but like SearchWP Live Ajax Search, it also works excellent with Relevanssi.
This plugin is an easy way to add a search feature to your navigation menu or a site header. I’m using it here on Relevanssi.com: click “Search” in the navigation menu, and the search form modal will pop up. The search form also works great on mobile devices.