If you have installed the free barcode extension of YBM you can download and unpack the “Email Format Template for Picklist”. You copy the contents of the text file, for example into a new transactional email template in the backend of Magento. You are welcome to make further changes here. The picklist you see in the pictures for this product corresponds to this template.
Then you have to set the template created by you under Configurations-> Sales-> Sales Emails-> Picklist.
The order number should now already be displayed as a barcode.