The MsgTo command brings up the "Send message" dialogue box. The parameters for this command allow you to specify the WMID of the message's recipient, as well as the message's subject and text.
Write to me by WM mail
<a href="wmk:msgto?to=0636776357492&subject=Hi!&msg=Just a test :)&BringToFront=y">Write to me by WM mail</a>
See also: WMK ProtocolRefreshPayToDisplayExit