Welcome to the community forum.
You click/tap the search icon on the page you get and search for “unlock” 
. Apparently they set up a new page for this …
https://support.fairphone.com/hc/en-us/articles/360048646311
Be aware that installing TWRP is not the recommended and standard way to use TWRP on the Fairphone 3, as installing TWRP there means altering the boot partition. Usually you would just boot TWRP with fastboot when needed … Used to the Fairphone 2? Don't do these Fairphone 2 things on the Fairphone 3!
For Fairphone OS installing TWRP on the Fairphone 3 will break OTA updates, for /e/ this “only” results in /e/ OTA updates restoring the recovery that comes with /e/.