Replace get_search_form

wpsearch.php

 // Search Form
    function vn_wpsearch($form) {
        $form = '<form role="search" method="get" id="searchform" action="' . home_url( '/' ) . '" >
        <label class="screen-reader-text" for="s">' . __('Search for:') . '</label>
        <input type="text" value="' . get_search_query() . '" name="s" id="s" placeholder="'.esc_attr__('Search the Site...').'" />
        <input type="submit" id="searchsubmit" value="'. esc_attr__('Search') .'" />
        </form>';
        return $form;
    }

    add_filter( 'get_search_form', 'vn_wpsearch' );