Hey Brian,
Try wrapping your payload using the "member" key. Something like this:
{
"member": [
{
"_action": "AddChange",
"orgid": "ORGDER",
"company": "BPTEST1",
"name": "Testing from API",
"paymentterms": "N30",
"disabled": true,
"type": "V"
}
]
}
Side note, the query parameters that you're using (e.g. Content-Type and x-method-override) should be passed as HTTP headers along with the API Key.
Hope this helps,
Alex