Custom Search Form HTML

<form method="get" id="searchform" action="<?php echo home_url( '/' ); ?>">

        <p class="search-box">

    <input type="text" name="s" id="s" placeholder="Search..." minlength="4" required>

    <!-- Prefill with category slug eg. 'events' -->

    <input type="hidden" name="category_prefill" value="events" />

    <input type="submit" id="searchsubmit" value="Search" />

        </p>

</form>