You have two choices: look at some examples and decide something arbitrarily, like "the first byte is ASCII, the second byte is zero," or you can actually find out what the application is expecting. The former can get you running, the latter will avoid sticky problems when a message must include non-ASCII characters like u-with-umlauts or capital-sigma or elvish-parma.

If you find the actual *encoding* standard they expect, you'll probably find a Perl module that will help you encode to that scheme without much fuss. It would be a rare standard that forced you to encode things yourself, but the perl builtin functions pack and unpack are a good start to your solution.