No, you should not buy a ticket. You have to check in and out with your OV-chipcard.

If other people travel with you, they can buy a ticket with 'meereiskorting'

but i have already bought a Meereisretour ticket, why i cannot use it with a paid subscription?

Because it is meant for people who are travelling with a subscription-holder. You are the subscription-holder and have to check in and out with your OV-chipcard. With an e-ticket it is not possible to see if you started travelling in peak- or in discount-hours.

clear enough, thank you!