Default get mount point for specific root location

Hi, I am doing na updater-script for flashable zip package, but I am stuck on certain point:

I would like to make changes in following location:
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so I need to execute (in my script)
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run_program ("/sbin/busybox", "mount", "/?????");
what I need to know how can I determine what to mount to get access to the /res/customconfig/, e.g. if there is a command I can use to determine using terminal etc ...

Thanks for answer
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