Open your theme.liquid file in the online code editor, go to line no 61 and add the class site-header__search-input--visible
to the search box input, like so:
<input type="search" name="q" id="SiteNavSearch" placeholder="{{ 'general.search.placeholder' | t }}" aria-label="{{ 'general.search.placeholder' | t }}" class="site-header__search-input site-header__search-input--visible">
You'll need to scroll to the right, it's a long line of code:
This does work but if you click outside of the search box it will disappear again because the "hide" script hasn't been disabled. To disable this, you need to also access the theme.js.liquid file and comment out (or delete) the .removeClass script as shown:
Hopefully this might help.