I did find a way, but it's really not the right way; it seems the software lacks this feature (which seems pretty basic to me!). I found a slightly newer version (8.6.3) and it still didn't have a way to do it.
The way I found was to put a visual basic script (file.vbs) in my startup folder that waits until personal communicator opens (checking every 1 second). Once it opens, it waits a minute then closes it (which sends it to tray). That 60 seconds is to allow time between when CUPC first shows up and when it's done logging in etc (and minimizable) - which can take that long on my computer with all my other startup stuff completing too. The problem is the delay isn't consistant so this doesn't always work perfectly. This was my first time trying to do something like this and had never used vb so I was kinda just guessing at the code from examples, if someone knew vb they could probably write something more effective without using delays.
dim sh
set sh = createobject("wscript.shell")
Do Until sh.appactivate("Personal Communicator")
wscript.sleep 1000
Loop
wscript.sleep 60000
sh.appactivate("Personal Communicator")
sh.sendkeys "% c"
set sh = nothing
