One way is to copy the text including the line break (or just the line break) from notepad or some other such program and then paste it into the custom property. I don't think there's a way to enter the line break directly.
Hold down the alt key, punch in 0010 on the number pad and release the alt key. See thread559-147251 and thread559-131040 for discussions about other symbols which can be entered in this manner.