Thanks. I overlooked this in the config file. I should have known...
It's in the autostart.sh file you need to look.
/home/user/.config/openbox/autostart.sh file is the full path to it.
* autostart.sh : where all is configured to get programs started automatically at login. (Read all the comments, lines starting with '##' or with '#' and you will know much very easily).
* menu.xml : is main menu tree (the one that you can modifye bye hand or with "obmenu" : except that when I launch obmenu I don't understand how that works ! /o\
* rc.xml : contains default configurations, and one part with "personalized configurations" that you will find while browsing in it. Once used, it's quite self explanatory.
You will find it at the line starting from " <!-- personalized config -->".
I invite you to read throught the rest as well, then you may find yourself at ease to make it your own.
In case you damage it,in the worse case you will be able to restore it by getting a copy of the former config file which is in /etc/skel/.config/openbox directory (if needed to restore from there check permissions once the file restored to it's relevant home directory)
If you change menu.xml or rc.xml, you need to reload openbox to get the changes at once : with the right-click menu 'Openbox > Reload Openbox' or, with the command line as user: 'openbox --reconfigure'.