Hello Whaimboayat36
You simply have to activate your Swiss Travel Pass Flex before boarding a public transport service, and then show the QR code together with your physical passport the conductor.
There is no need for a customer number. This would be required for the SwissPass, a resident offer. Your ticket is named Swiss Travel Pass.
Whether you need to reserve seats? I answered that question to a fellow traveler in this thread:
https://community.sbb.ch/d/23170-purchased-a-sbb-6-day-1st-class-rail-card-cant-reserve-tickets
Let us know if you need more assistance.
Kind regards,
Roland