[Serusers] How set up proxy to process messages

Hi everyone, I have problem with set up proxy to process message INVITE. I have 2 SIP users of different domain. How can I set up proxy for each domain to process message INVITE from one domain to another? Now when user 1 of domain A send INVITE message to user2 of domain B then message go to direct to server of domain B, do main route{} block of B and then send to user2. I need the INVITE message of user 1 the first go to proxy of domain A, process that message (do main route{} block) and then send that message to proxy of domain B, process it (do main route{})and then send to user 2. Thanks, Tuan.