I wanna add search box in the menu bar.

Hey there! Expound does not allow you to add a search bar to the navigation bar by default, but you can probably add it with some custom CSS and/or an action. Check out the expound_navigation_after action. I used it to add some social links to the right side of the bar on wpmag.ru, you can probably use it to add a search box, then style it with some CSS.

Hope that helps!