Hi All,
So I’ve finally got my barcodes printing on my Custom Print Formats and I thought I’d share 
Using this site:
http://www.barcodes4.me/apidocumentation
And their REST API you simply need to call the barcode as an image in your HTML.
For me I wanted a QR code representing the URL of the Sales Order so we can scan this into a browser and have the Sales Order come up onscreen within the production workshops.
So my code is:
<img src="http://www.barcodes4.me/barcode/qr/myfilename.png?value=https://[company_name].frappecloud.com/desk%23Form/Sales%20Order/{{ doc.name }}">
Note I’ve replaced our real company name with [company name] in the URL for anonymity.
Also note if you need to return any special characters - ie the # within the URL like I had to between “/desk” and “Form” you will need to use the ASCII encoding Reference value. For “#” it’s %23, [space] is %20, etc. A full list is here:
PS: This might be a good knowledgebase topic - feel free to modify my description above or suggest any changes I can make to ensure it’s helpful for everyone.