Open Lilybot is designed to work just like regular Lilypond does from the terminal. That means to compile some Lilypond code, you should type $lilypond \relative { c' d e f g a b c } . You can’t upload a Lilypond file due to security reasons, that isn’t the purpose for this bot anyway.
Extra options can be passed as usual (with the exception of --output). Note, however, that only the most recent file is uploaded! That means that using $lilypond preview ... will not upload a PDF and a PNG file, but only a PNG.