LTM forward proxy in Route domain?

Are you referring to creating a virtual on network A doing a simple FastL4 to a secondary route domain? The content help within the BigIP interface does give some good basics of creating/attaching route domains to networks.

Beyond our documentation, there are a lot of good question/answers within the DevCentral community. Here's one that answers a lot of questions: https://devcentral.f5.com/questions/route-domains-24086

What are you trying to accomplish specifically?