OLD:

The old structure only required you to enter a status message, which you sent to the home.

http://twitter.com/home?status=I%20am%20doing%20this%20thing%20on%20Twitter%20@arvanhalleorg%20http://www.test.com/myurl/

NEW:

The new structure requires that you provide, at a minimum, a url, via, and text.
(note that the url and via are appended to your message, so you don’t need to add them to your text)

http://twitter.com/share?url=http://www.test.com/myurl/&via=arvanhalleorg&text=I%20am%20doing%20this%20thing%20on%20Twitter

The url will be shortened by Twitter’s internal API and @arvanhalleorg in this case, would be appended to the text.

Output is something like this:

“I am doing this thing on Twitter http://t.co/pTbu5c0 via @arvanhalleorg”, which appears in a tweet box.

No automatic tweet. This allows you to adjust the message, before blindly putting weird things on your timeline.