You can't - it converts the message to RTF formatting so html gets message up. plain text is usually betwee, but not always. At least its easier to read.
You can right click and drag to create a task with an attachment - or create the task your usual way then drag the message to it to add it as an attachment. Not quite the same, but at least the formatting is perfect when its opened. I probably have a macro here somewhere the creates the task with the message in the body and as an attachment so its one step.