Before the flight, you can purchase a voucher in the Lufthansa/Telekom Ground Shop, which you can then redeem on board via the FlyNet portal. Click here to go to the ground shop: Lufthansa Ground Shop (telekom.de).
If internet is not available on your flight, you can also redeem the voucher on a future long-haul flight with the Discover Airlines or Lufthansa FlyNet service on board. The voucher is valid for the calendar year of purchase and the three following calendar years and can only be redeemed once on a flight segment (e.g. Frankfurt-Windhoek) with the Discover Airlines or Lufthansa long-haul fleet equipped with the FlyNet system.
Vouchers that you have received elsewhere (e.g. as part of promotions) may have different terms of use or validity than the tariffs on board or in the Ground Shop.