Note: As of IPFire 2.27 - Core Update 160 you must use the token created in V2 interface of FreeDNS. FreeDNS released Version 2 of the dynamic update interface on 17 February 2016 and Version 2 is now implemented on the IPFire DDNS system.
To enable Version 2, go to https://freedns.afraid.org/dynamic/v2/ and enable the needed subdomain.
Click Action: Enable Dynamic DNS... and then click Apply.
Note: Every time you click Action: Enable Dynamic DNS... and then click Apply the FreeDNS system will assign a new token. And a token change will be needed on the IPFire Dynamic DNS page.
Now make a copy of the token. It is needed for the IPFire Dynamic DNS.
And add the FreeDNS subdomain and token to the IPFire Dynamic DNS page at menu Services > Dynamic DNS:
Click Add and Instant Update. The FreeDNS Hostname should appear in green.