What the setting does: the Project OS page of a site has a switch, "Send tasks to Project OS", and a direction, either send only or send and accept a request to start work. SerpUp stays the system of record. Only the workspace owner can change the settings, and the page shows the last sync time, the last error and any changes waiting for approval.
Where the connection lives: the address and token for Project OS are server configuration of the deployment, not fields on this page, and the workspace must be listed there. If your deployment is not configured, ask whoever runs it.
Current limit: in this release the live Project OS client is not built, so turning the switch on records your choice but does not send tasks. A configured deployment reports that in the job ledger instead of pretending to sync. Check the "Sync status" box on the page: "Last sync: never" means nothing was sent.