If you've removed the search form from the menu, go to Layout > add a HTML/JavaScript widget to the Search Form (Menu) widget area > paste this code inside:
<form action="/search" class="menu-search" method="get">
<input class="search-input" name="q" placeholder="Search..." type='text' value=""/>
<button class="search-submit" type="submit" value="Search"><i class="fas fa-search"></i></button>
</form>
And make sure this is in the "Social Icons" widget in the Social Icons (Menu) widget area, so the search form can be accessed:
<span class="search-toggle"><i class="fas fa-search"></i></span>
Add it directly above the </div> at the end.