The Imperium Nova forum software does not allow posts to be truly anonymously. However, as there are many situations where an anonymous post may be useful for roleplaying purposes, certain players have taken it upon themselves to act as middle-men for people who wish to remain anonymous.
To make an anonymous post, you must first contact an anonymous poster in your galaxy. These fine men and women have sworn to keep your identity confidential, no matter the cost, and to never take action on the information you have given. In an embassy message, send them the text of the anonymous message you wished to have posted. The anonymous poster will then post it on the forums.
All anonymous posts begin with (Anonymous). If the post begins a new thread, it will be in the title. If not, it will be at the beginning of the post body. Observers are reminded that these posts are not necessarily attributable to the anonymous poster. They merely act as a middle man.
Just do the above in reverse - whenever someone asks you to post something anonymously, do so, label it (Anonymous), and promptly forget who it was. Remember that your privileged knowledge of the true identity behind the anonymous post is out-of-character information. You should not give out this information to anyone else, and in-game you should proceed as though you did not know it.