Where to put the script to custom some environment in debian installer?

[Magicloud Magiclouds <magicloud.magiclouds@gmail.com>]

Hi,
  When using debian installer cd, there are many options to set. And I
could modify some parts, for example localechooser, to make some
customizing.
  What if I just want to make some options fixed, so the user do not
have to use localechooser at all? I cannot find where to put the
scripts to set something when the installer starts.
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