Create a file named searchform.php and place this in the root of the theme folder. This will replace the default WordPress search form.

<form role="search" method="get" class="search-form" action="<?php echo home_url( '/' ); ?>">
    <input type="text" placeholder="<?php echo esc_attr_x( 'Search website...', 'placeholder' ) ?>"
            value="<?php echo get_search_query() ?>" name="s"
            title="<?php echo esc_attr_x( 'Search for:', 'label' ) ?>"/>
</form>