custom-mode-hook

custom-mode-hook is a variable defined in `cus-edit.el'.
Its value is

nil

• This variable is an alias for `Custom-mode-hook'.
• This variable is obsolete since 23.1;
  use `Custom-mode-hook' instead.
• This variable may be risky if used as a file-local variable.

Documentation:
Hook run after entering Custom mode.
No problems result if this variable is not bound.
`add-hook' automatically binds it. (This is true for all hook variables.)

You can customize this variable.