You'll have to update your Cisco to the latest firmware first (10.4.1 SR2 or higher) - which is done via downloading the firmware from a TFTP server. Without the latest firmware, these options won't appear!
The syntax you'll need is fnc=blf+sd;sub=*extension number*@$PROXY;ext=*extension name*@$PROXY
For example - if Bob was extension 1001, the syntax would look like this: fnc=blf+sd;sub=1001@$PROXY;ext=Bob@$PROXY
Once you click Save changes, you'll then see it light up on your 8861 if done correctly! If not, then check the syntax and the extensions you have entered and try again!