Overnight train from Madrid to Lisbon

I come from Hong Kong planning to visit Madrid, Lisbon, then to Andalucia and back to Madrid at the end of June. However I find that I cannot book/buy the overnight train from Madrid to Lisbon from Renfe's Tiknet system. As we plan to take the train the next night after our arrival to Madrid, I may not buy tickets of second class with couchette. Can anyone suggest any method so that I can book/buy the ticket before our arrival