? whitelist for POST requests

Hello All,
I am getting this in my browser when trying to set up reverse proxy
in my Apache2 server.
The destination site needs login/password, and somewhere I must allow
it to POST.
However, searching through Google and Apache docs hasn't brought too
many ideas.

"In order to accept POST request originating from this domain, the
admin must add this domain to the whitelist."

So, where's this magic white list? Is it a file? Is it a directory? Is
it a TWISTER? :-)
Thanks!