Custom headers need to be created for log parsing if the log file does not contain header fields or if you are using custom-csv parser.
Configure the custom header by entering: set log-upload header <custom header>
Here is a custom header for proxysg-http-main format:
set log-upload header "<parser name>::#Fields: date time time-taken c-ip cs-username cs-method sc-bytes cs-host cs-uri-port cs(User-Agent cs-bytes sc-filter- result"
Test with a small sample file to ensure that the events are getting extracted before streaming the syslog messages.