The SSID would be configured on the AP. OPNsense wouldn't even know it's there.
How many devices (APs and clients) in the network? Do you need central management?
Not sure where the hangup is here - you configure the SSID on the AP, because the AP is providing WiFi service. OPNsense has nothing to do with it. I don't know the Synology stuff, but from a YouTube video that a quick Google search found, it appears that you would create or modify a "Local Network" under "Network Center" and specify your SSID there.