add a message field “my_message_field”
Add an acf/prepare_field filter for the message
add_filter('acf/prepare_field/name=my_message_field', 'prepare_my_message_field');
function prepare_my_message_field($field) {
//set message based on another field
$field['message'] = get_field('my_other_field', 'options');
return $field;
}