After the great meal I had on my Korean Air flight from Seoul to LAX, I thought it would be nice to give them a compliment for it. After some time, I did in fact find a form on their website to make a compliment. However, that form only would work if I had a Korean Air Skypass membership. I tried filling in the membership number with all zeroes, all ones, etc to no avail.
The wording on their form makes it sound optional, but the form will not submit without a valid number. I haven’t had that problem with any other airlines, as I have complimented the crew of flights on other airlines before through their websites.