To start / stop RPFWD (or any interactive based protocol), use the SendMythicRPCProxyStart and SendMythicRPCProxyStop RPC calls within your Payload Type's tasking functions.
For RPFWD, you want to set LocalPort to the port you want to open up on the host where your agent is running. RemoteIP and RemotePort are used for Mythic to make remote connections based on the incoming connections your agent gets on LocalPort within the target network. The PortType will be CALLBACK_PORT_TYPE_RPORTFWD (i.e. rpfwd).
In general, your agent will open up LocalPort on your target machine. When it gets a connection to that port, it'll generate a random uint32 to identify that connection, and forward along what it gets from the connection to Mythic along with that identifier. Mythic will get that information and open a new connection to RemoteIP:RemotePort and forward that data along. At that point, both sides are just forwarding data back and forth. Eventually, one end of the connection will terminate and at that point a final message will get sent to tell the other side to also close.