default-composer-hdrs are the headers that appear by default upon
invoking the composer command. Any Rich Header or Customized Header can
be added to this list, but the header must already exist in Rich Header
or Customized Header. To add a default value to a composer header, you
must also add it with the default value in the Customized Headers list.
I would urge you to make sure the first letter is capitalized AND there
is a space between : and the header value, as it makes it easier to
read. That space is required to compose an article for a newsgroup, if
you use alpine as a newsreader.