Something like this
add_filter('acf/prepair_field/name=MESSAGE_FILED_NAME', 'change_message_MESSAGE_FILED_NAME')
function change_message_MESSAGE_FILED_NAME($field) {
$some_value = get_field('some_field');
$some_other_value = get_field('some_other_field');
$field['message'] = $some_value.' '.$some_other_value);
return $field;
}