How do you add a barcode to an envelope in Pages?

Unlike Word, Pages does not offer this feature as no barcode font ships with macOS. If you enter barcode in the Mac App Store search field, you will find plenty of apps to do this, and provided they generate the correct postal bar code for your country, you can manually add that to an envelope in Pages. If you do, it should be as a Pages template to reduce the workflow with subsequent envelopes.

Depending on exactly what you’re up to here…

https://www.fontsaddict.com/font/category/Barcode

If USPS:

https://postalpro.usps.com/mailing/encoder-software-and-fonts

https://postalpro.usps.com/mnt/glusterfs/2021-07/IMb%20Font%20Download_7-29-2020%20v1.pptx

etc…