User number

I'm trying to enter a command to send a message to a user:

/mod/messages/send.php?send_to=%USER%

This actually sends a message to oneself.  The way the normal message url displays in Elgg is:

/mod/messages/send.php?send_to=2565

Since %USER% doesn't work with the messaging system to call up the user number, is there something else I could put in its place to display the user's number instead of their username?