config.php

<?php

// D7
// variable_get('webform_default_from_address', variable_get('site_mail', ini_get('sendmail_from')));

// D8
$site_default = config('system.site')->get('mail', ini_get('sendmail_from'));
config('webform.settings')->get('default_from', $site_default);


// webform.install
function webform_install() { 
  $site_default = config('system.site')->get('mail', ini_get('sendmail_from'));
  config('webform.settings')->set('default_from', $site_default)->save();
}