Use this code example to add custom HTML to the conversational form start page below the start button.

add_filter('frm_chat_start_page_content', 'frm_chat_add_html', 10, 2);
function frm_chat_add_html( $start_page_content, $args ) {
	$target_form_id = 310; // change 310 to your form ID.
	if ( $target_form_id === (int) $args['form']->id ) {
		$start_page_content .= '<br>';
		$start_page_content .= 'Additional start page content';
	}
	return $start_page_content;
}