Client B must enter the ip address of Server A. If they are both on the same box then it’s 127.0.0.1 which is localhost.
If they are both on the same subnet, Server A must look at his ip address that is connected to the LAN.
Client B must use that ip address to connect.
Both Server A and client B must use the same port.
if Server A has started his server on port 25000 then client B must connect to port 25000