add_user_header configuration option not working?

Hi,
I've got a question regarding the add_user_header configuration option.

I'm using Apache Apollo v 1.7 with STOMP over WebSockets and
authentication enabled. Now I want Apollo to add a header to incoming
messages with the sender's authenticated user id. I've added the
add_user_header configuration as discussed here:
https://activemq.apache.org/apollo/documentation/stomp-manual.html#Stomp_Protocol_Options.

However, the requested header is never added to incoming messages (but
I've verified that users are actually authenticated). Any suggestions
what I'm doing wrong here?

Thank you!
Dennis

This is my configuration file: