BTW, it was a problem with how I was passing the parameters to the API.
All fixed.
Also, if anyone is in need of a dynamic dns script (private server, not like dyndns.org), let me know. I may post it in the wiki.
We have a MT that, for some reason, the standard dyndns script we have does not work. We run the exact same script on 12 other routerboards, but this one fails. All version 3.13. So, what we do is tell the mikrotik to grab a php page from one of our public facing servers. Then, that php script runs an api request against a second mikrotik that DOES run the dyndns script properly. The second MT updates the dynamic dns entry for the first MT.