What do I put in the generator-settings area in server.properties?

So I'm trying to make a custom superflat world on my server. The wiki says that I need to put flat as the level-type, and I need to modify the generatorName, generatorVersion, and generatorOptions. The thing is, there's no more info on this, although I think it might be important. But for now, I found this thing called generator-settings line in my server.properties that was originally blank. The wiki says that it is "The settings used to customize world generation. See Superflat and Customized for possible settings and examples." But there is no more info about it in the superflat page. So what do I put there? I've tried doing the preset but that doesn't seem to work. Am I doing something wrong here?

This feature is currently (since 1.13) completely broken. Whatever you put in, it's completely ignored. Here is the bug report https://bugs.mojang.com/browse/MC-134900

For now you have to create the world in Singleplayer, then copy it over to the server. Make sure to name the folder "world" (or edit the level-name property).

Update for 1.16: The previous bug was supposedly fixed, but it still doesn't work, instead the server now crashes even with a valid input, because it actively edits the file to contain invalid input for some reason. I created a new report for that: https://bugs.mojang.com/browse/MC-182421