As Peter described, user authentication for HTTP station is currently not supported by the firmware because it requires additional parsing and modifications to the switch_httpstation function. I am curious what kind of ‘relay board’ that you are using requires authentication?
An alternative (albeit a bit ugly) way to achieve this is to use a local server to do the translation, basically, have OpenSprinkler send a command to the server, with user name, password encoded in its On/Off command, and the server will translate it to the appropriate HTTP command and send it out. This server can be done using a raspberry Pi or any Linux box, and you can write the script in Python, nodejs etc.