Looks like you can use ZPL with that printer, my suggestion would be to generate a ZPL string instead. Head over to Labelary Online ZPL Viewer and you can see an example.
See this post as using system.net.httpPost to send it to the printer.
The printer may or may not accept a print with this command, Zerbra branded printers work great, but I've have printers that emulate ZPL not work with this script, but I was able to use the TCP/UDP driver to connect and send the ZPL through that instead.