As I understand from the letter I should use my cos number for sponsor licence number. However, when I tried to fill the application forms at https://visas-immigration.service.gov.uk , the below warning occurs.
Check your answers:
• licence number not found - please check the number. Enter your employer's sponsor licence number, not your Certificate of Sponsorship (CoS) reference number.
Am I doing wrong? Which cos number should I use?

Would you please help me to solve the situation and fill my forms to extend?
Many Thanks