The ONLY solution to this topic is:
1- You send a message to your guest to inform that you will decline the request in order to send a special offer
2- You DECLINE the request
3- A window pops up asking for the reason, UNBLOCK the dates and confirm
4- You open again the conversation with your guest in the inbox section
5- The SPECIAL OFFER will be at the bottom of the Reservation Details just after the price
It happen to me and I called the customer service, they were also struggling in giving an answer...