I guess I found a way to make it work.
Add the following lines to httpd.conf in apache proxy server.
LoadModule proxy_module modules/mod_proxy.so
LoadModule proxy_http_module modules/mod_proxy_http.so (This is
required to promt username/password - defined in internal IS)
ProxyPass /submitToPartner http://host:port/invoke/wm.tn/receive
ProxyPassReverse /submitToPartner http://host:port/invoke/wm.tn/receive
where host:port is your IS host name and port.
#webmethods-Protocol-and-Transport#Integration-Server-and-ESB#webMethods