I needed to get a value passed in from a proxy server via the HTTP header. I’m using Oracle HTTP server.

Browsing the OTN Application Express forums, I learned that do this, one has to:
1. Edit the dads.conf to include the following line.
PlsqlCGIEnvironmentList [your variable name]

2. Add code to obtain that variable. I added a Display as Text item and made its source a PL/SQL Function or Expression.

The source value was:
OWA_UTIL.GET_CGI_ENV(‘[your variable name]’);

Note that the following will print out all environment variables.
You can put this code in a region of type pl/sql block, or as an item.