If you need to add a barcode to your document, you can do so by embedding an image (for specific instructions on embedding an image in your template, use the help articles here)
To create the barcode image URL, we're going to use a free service at https://www.barcodesinc.com/generator/index.php
On that page you can build a barcode, then after you click "Generate Barcode", you can grab a "link" to the bar code image:
In the Link (the URL to the barcode image), we can use merge fields for the value of our barcode like this:
The last step in the process is to use this image URL and embed the image in our template. Again, for specific instructions on how to embed an image in your template, please use these help articles.