Easiest way is to do it through their res call center, which is not that easy!!

If you want to do it online, you can select the name from the drop down list next to pax name / info, if that family member is already added to your account as a household head account. They claim this is done to prevent fraudulent redemption.

If member is not already added, you need to create a Fursan account in their name first, then add it to your account (cumbersome process through "manage family members and invite family members links when you are logged in), that way you can redeem for them and also get miles flown by all added members collected in one account.