Replace menu item text with search form

menu-item-search-form.php

<?php
/**
 * Allow search form in menu items. (add CSS class has-search-form to menu item classes)
 */
add_filter( 'walker_nav_menu_start_el', function ( $item_output, WP_Post $item ) {
	if ( isset( $item->classes ) && in_array( 'has-search-form', (array) $item->classes ) ) {
		$item_output = preg_replace( '/(<a.*<\/a>)/', get_search_form( false ), $item_output, 1 );
	}

	return $item_output;
}, 10, 2 );