Adding Line Break into Post

I wrote a function which saves a post using the DataWriter. Everything works fine except those added Linebreaks in the message.
I wanted to append two line breaks and add something like "- sent with...".
What should I add to the messagestring to save real linebreaks?
I tried
[br], <br> and \n
but on the forumpost it displays that raw code instead of the linebreak.

Any suggestions? :S