lua dissector bind a Protocolfield treeitem to existing protocol in wireshark

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Hi I want to register my subprotocol field into an existing protocol such as iPerf2. In my lua script I registered my protocol as post dissector and I can access existing fieldvalues like iperf2.client.bandwidth with Field object defined outside of dissector but I cannot add a tree item to an existing Iperf2 tree. When I first add a tree in dissector function a new tree comes under Iperf2 root tree. What I prefer is this tree is binded under existing iPerf2 tree. Is there a possibility for this? I searched a lot and could not find a solution. Thanks for helping in advance.