<header>
<!-- Other header content -->

<form role="search" method="get" class="search-form" action="<?php echo esc_url(home_url('/')); ?>">
<label>
<span class="screen-reader-text">Search for:</span>
<input type="search" class="search-field" placeholder="Search …" value="<?php echo get_search_query(); ?>" name="s" />
</label>
<button type="submit" class="search-submit">Search</button>
</form>

<!-- Other header content -->
</header>