How to create a rule to allow external http traffic to internal webserver at port 8888 using MS Forefront Threat Management Gateway?

Question

I am using Microsoft Forefront Threat Management Gateway(TMG) as a firewall for my network.

I have a web server running IIS and Apache. IIS is running on port 80 and Apache is running on 8888.

I wanted to allow all external HTTP traffic to webserver(192.168.1.25) on port 8888 (i.e., Apache).

I am trying to create a rule from TMG but didnt find the option to use custom port on tab "to".

Please help me, create a rule to allow external http traffic to apache at port 8888

regards sia