How do I add Flex to my ticket?

You can add Flex when booking a flight. Once you have selected a Basic ticket, you fill in your passenger details in step two. Below, you add Flex to your booking with one click. You can only do this while making the booking and not afterwards. See how it looks like:

Good to know: When you select Flex, you automatically purchase it for the outbound and inbound flights and for all passengers you are travelling with.