Short answer you can't. Just create a new account with throwaway email and then create that post, no one will know if it was you.
Hahaha… This is quite straightforward, but it's true. I can imagine the number of newly registered accounts that will make just a few posts before going inactive if everyone keeps creating new accounts in this manner. Is that the reason certain newbie accounts occasionally post about seemingly strange topics? The only option to remain anonymous is to create a new account and post whatever you like; no one will ever be able to identify you. There are no longer any settings that allow for anonymous posting.
Creating a new account in order to make a post that is either controversial or that you wouldn't want to get you into trouble is even encouraged by theymos himself.
Unlike what some people says that it is an act of cowardice, I believe it has more to do with privacy instead.
It is important anyone that wants to post information that they don't want to be linked to their main account, especially this time around that the reputation board is full of drama and people fighting against others.
Before now people request theymos to change their particular post to Anonymous and he did for them, but I don't know if it still exist till date.