Add following code in your theme’s (child theme’s if you are using) functions.php

add_filter( 'fep_form_fields', function( $fields ){
	$fields['cus_fep_checkbox'] = [
		'type'     => 'checkbox',
		'where'    => 'newmessage',
		'priority' => 22,
		'cb_label' => 'Can we use your message on the site as a story?',
	];
	return $fields;
});

add_action( 'fep_action_message_after_send', function( $message_id, $message, $new_message ){
	if ( ! empty( $message['cus_fep_checkbox'] ) ) {
		fep_add_meta( $message_id, 'cus_fep_checkbox', $message['cus_fep_checkbox'], true );
	}
}, 10, 3);

add_action( 'fep_display_after_message', function(){
	if ( fep_get_meta( fep_get_the_id(), 'cus_fep_checkbox', true ) ) {
		echo 'We can use this message on the site as a story';
	}
});