Redefine <return> to do newline in Custom mode

[Matthew Flaschen]

By default, hitting return in custom mode goes to the next field.
However, I would like to rebind it to newline to make it easier to edit
multi-line fields.  I have tried:

(define-key custom-mode-map "RET" 'newline)

as well as several variations ("^M", "<return>, etc.)

but it seems to have no effect in the actual customization field.  I am
running 22.1.1, packaged by Fedora.

Thanks,

Matt Flaschen