um ..... with a lot of luck
Enable chat in your options or preferences. Right click on a file connected with your chat "victim" Select Chat. If a dialogue box appears. The rest is straightforward ... welll very often you type a message and then see "Host unavailable" Often no box appears ...
The system is very hit and miss and depends on the client the host is using, firewalls number of up and downloads going on, firewalls etc. Many people who actually do connect and start a chat get so excited that they hit the Block host button by mistake and ruin it all LOL
So It works like you might well have guessed ... only, sadly, not very often