I am trying to use the API (I want to test it so I can make a script that does this) to add a TXT record to my domain. I am having no luck doing so. I tried following the information here
https://www.dynu.com/en-US/Support/API#/dns/dnsIdRecordPost
but it didn't work. I then found this
https://www.dynu.com/resources/api/documentation
and noticed it was different but that didn't work either. I have tried as many variations as I can, and I keep getting
{"statusCode":404,"type":"Request Exception","message":"Invalid."}

Could someone please help me with this?

More Info:
I have tried this:
{
"textData":"TestingString",
"domainName":"mydomain.com",
"nodeName":"testTXT",
"recordType":"TXT",
"ttl":120,
"state":"true"
}

and this:

{
"text_data":"TestingString",
"domain_name":"mydomain.com",
"node_name":"testTXT",
"record_type":"TXT",
"ttl":120,
"state":"true"
}

as bodies for the API call, but still I get the same issue

The correct parameter format is

"textData":"TestingString",
"domainName":"mydomain.com",
"nodeName":"testTXT",
"recordType":"TXT",
"ttl":120,
"state":"true"

You can first try with the webpage tool to post data. Once successful, you can then work on your own script. If you still have issues, open a support ticket with the detailed script you posted.