The instructions are at the side of the message box when you type.
Links The simplest way to insert a link is to enter the link itself, surrounded by and . So if you type www.mumsnet.com, the link will display as www.mumsnet.com. If you want your link to display text other than the web address itself, leave a space after the address then add the text before the ]]. So "Look at this page", would display "Look at this page".