MappedPort is a published property that represents the port number to use for outbound connections in the mapping proxy server. 

Values in MappedHost and MappedPort are used in the Host and Port number for the connection opened to the remote system when a listener thread accepts a new local client connection. MappedHost and MappedPort are assigned to the Host and Port values for the OutboundClient in the TIdMappedPortContext created for the proxied client session. 

Assign values for the MappedHost and MappedPort properties prior to setting Active to True and starting the listener thread(s) for the server.

TIdTCPClient TIdTCPClient.Host TIdTCPClient.Port