It can’t, at least for Code 128? There’s a text field that you enter the text into, and then the start/stop/checksum characters are computed.
replyIt seems like it doesn't do this for Code 128 (possibly because it is variable-width?). It definitely works with EAN13 though - I tried it locally using only the TTF file.
replyOh, interesting! I tried it in Word on macOS but didn't get it to work. But it works in the browser (question mark = calculates check digit).
replyIt uses this, which i have no idea what it is :-) https://learn.microsoft.com/en-us/typography/opentype/spec/f...