wp_admin_bar_search_menu

function wp_admin_bar_search_menu( $wp_admin_bar ) {
	if ( is_admin() ) {
		return;
	}

	$form  = '<form action="' . esc_url( home_url( '/' ) ) . '" method="get" id="adminbarsearch">';
	$form .= '<input class="adminbar-input" name="s" id="adminbar-search" type="text" value="" maxlength="150" />';
	$form .= '<label for="adminbar-search" class="screen-reader-text">' . __( 'Search' ) . '</label>';
	$form .= '<input type="submit" class="adminbar-button" value="' . __( 'Search' ) . '"/>';
	$form .= '</form>';

	$wp_admin_bar->add_node(
		array(
			'parent' => 'top-secondary',
			'id'     => 'search',
			'title'  => $form,
			'meta'   => array(
				'class'    => 'admin-bar-search',
				'tabindex' => -1,
			),
		)
	);
}