WordPress doesn’t make adding a search form in the navigation menu easy. 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, like Twenty Twenty, include a built-in search feature in the header. 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 it also works great with Relevanssi.
This plugin can easily add a search feature to your navigation menu or a site header. The search form also works great on mobile devices.