Get command line if game was launched via Steam URL, e.g. steam://run/appid//command line/. This method of passing a connect string (used when joining via rich presence, accepting an invite, etc) is preferable to passing the connect string on the operating system command line, which is a security risk. In order for rich presence joins to go through this path and not be placed on the OS command line, you must set a value in your app's configuration on Steam. Ask Valve for help with this.