How do I tell Adagio to put the picking slip number in that optional field, and also to use the order number - release number format(123456-0)we are using?

You can't tell Adagio - you have to tell your Order Entry people. This "order #-release 3" is a process that you have set up to handle your circumstances, it's not part of the Adaigo OrderEntry "routine".

Once you've put the number in the optional field, you just need a specification to print it out.

BTW - OrderEntry V8.1B, due out in a week or so, will allow you to edit this information after an invoice is posted.