How to process Client side's TCP Payload after SERVER_CONNECTED?

You should do something in the line:

when SERVER_CONNECTED {
    TCP::collect
}
when SERVER_DATA {
   TCP::payload replace 0 [TCP::payload length] "Your string"
   TCP::release
}

It depends where you started the collect process, if you started it in CLIENT_ACCEPTED event it will collect the client side of data. You might also look into

serverside { TCP::collect }
clientside { TCP::collect }

to be more specific where you want the collection to happen