This answer is for android. I do not use any other Memento.
In Memento, the barcode field is for scanning barcodes. You can create an entry by scanning a barcode and you can find an entry by scanning the barcode.
You said you wanted to generate barcodes, so the rest of this answer is about generating a barcode from an Entry in a Memento Library.
You will need an app for generating barcodes. I'm sure there are several apps that would work. I like this one:
https://play.google.com/store/apps/details?id=com.teacapps.barcodescanner.proYou can make a barcode from a Memento entry using that
barcodescanner.pro app. Open the entry, then from the 3 dot menu choose "Send". (If your screen is wide enough it will be a share icon next to the 3 dot menu.) Then choose
barcodescanner.pro app. (It will show as Save File and as View code. Choose View code.) It will generate a QR code which you can then print or save, and the fields of the entry will be included as a caption to the QR code.