First thing I would like to say is I don't recommend automatic upgrades. If it isn't broke don't fix it. You may be making a new problem.
I don't believe you need a user name or password defined. You want it set to periodically. The interval is how often you want the obi to check for an update in seconds.
This is the current url of the firmware
BUT you should contact ObHai to see what If/Then logic may need to be added to the mix to make it work properly. Simply putting that url in might cause the obi to attempt unneeded upgrades that could slow the system.
http://fw.obihai.com/OBi-latest.fw I believe an automatic upgrade will cause the obi to make a graceful reboot afterwards. Which means it will reboot when a call is NOT being made.