Hi Sebastien,
There are two ways to do this.
1. via Serilog
If you're writing events with Serilog, you can enrich each event sent by the application with an appropriate property:
Log.Logger = new LoggerConfiguration()
.Enrich.WithProperty("ApplicationName", appName)
....
2. via API keys
Alternatively, if you have each application that connects to Seq use a unique API key, you can associate a property like "ApplicationName" with each key; go to Settings > API keys to see how this is done.
If you need more help with either method please let me know.
Best regards,
Nick