@bzh87 By using the following configuration, you can achieve basic load balancing of TCP port traffic between real servers.
1. Create a custom TCP service port for TCP port 3500 and call it as an HTTP service in the server policy.
2. Turn off HTTP parsing in the server policy in CLI by enabling 'noparse' setting (basically Fortiweb stops parsing the TCP data with noparse enabled)
But please note that Persistence doesn't work and System doesn't generate traffic logs, so it adds a little value (persistence + traffic logs for tshooting/future reference) compared to what typical Load Balancers do.
UDP load balancing is not possible on Fortiweb as it can't listen for UDP port traffic.