If you should ever come in the situation that you want a default custom field on your “write page” you should first of all create a draft with your default custom fields.
After that go to your phpMyAdmin interface and lookup the postid.
Once you got the postid simply goto “wp_postmeta” and find the postid of that draft. Change the id to zero.
Voilà now you will have the custom field you defined in the draft at your write page. This is especially handy if you always need the same custom field because you can simply copy it.