I just purchased my domain, I haven't transferred the other domain "yet" as I wanted to get this working first.

I recently moved off of clearos not by my choice have you but theirs, due to lack of support and depreciation of their current system 7.7.1.

I've installed PFSENSE as my primary firewall and have my current DNS for both domains pointed at my IP address however, I'm failing miserably trying to get the built in api in PFSENCE to check in for dynamic DNS services .

Any guidance you can provide would be greatly appreciated.

Probably a tad late but it is easy in PfSense
Log into PfSense
Services - Dynamic DDNS
On your browser go to https://www.dynu.com/en-US/DynamicDNS/IP-Update-Protocol
Find the entry you want. For example to update a domain to the IP you are on.
Find the form entry Update URL and enter the following obviously change username password and login details.
https://api.dynu.com/nic/update?hostname=your.domain.name&password=securepassword

I use a PfSense Router and it works great. Automagically changes the IP the instant your IP changes with your ISP.