Just an update - it did not work online despite I could see the choices. Then, I tried my luck and used the online chat with a live agent - he spent 30 mins and told me it‘s not possible to add because my flight is operated by LX but I corrected him that the flight is marketed by LX but operated by SQ, he apologized for the dis-information. I asked to have an email where I can escalate the issue, and he told him he will first give another try at his end. After another 40 mins, he was able to insert all my cook the book choices … I was prepare he would tell me still not possible, but to my surprise, he said everything done and ok. I checked SQ website on my booking and could see the system has reflected the choices correctly. Very interesting …