The ticket number starts with either IV or 15. It should not be confused with the booking reference. 

On printed tickets at home, you will find it at the bottom of the page. On the tickets withdrawn at the station, it is located on the lower right. 

Number beginning with IV
Simply enter the nine digits following the IV. 
 Example: IV 123456789 

Number starting with 15
Enter the nine digits following the 15. Disregard the last digit. 
Example: 15 123456789 0 

Number starting with 814
If your number starts with 814, it corresponds to the electronic ticket issued by your travel agency. The actual ticket number is on your boarding pass.