HTTP Event Collector: How to add Splunk request channel header in the URL programmatically?

I'd like to add the request header programmatically in Java with Logback. As surely there is no such thing provided by Logback, I wanted to use the alternative and add it directly to the header. Here's an extract of my logback.xml:https://myURL?channel=myGUIDmyTokensplunktestHEC5%date{"yyyy-MM-dd'T'HH:mm:ss,SSSZ"} - %msg However, I get the exception "ServerErrorException: -1, null". Does anybody know what I'm doing wrong? I appreciate your help.