REST request to Vertex Cloud

Has anyone had success using a REST request to the Vertex Cloud for sales tax calculations? If so, any advice? Vertex has been of little help, all their IBM i connections are SOAP.

I'm attempting to use Scott's HTTPAPI version 1.39, O/S version V7R3M0.

The debug log shows I am making the connection, but I am receiving either 400 Bad Request or 415 Unsupported Media Type , depending on what "Content-Type" is sent.
Testing the connection with Postman is successful.

Entire test program below:

http_debug(*on: '/tmp/debug.txt');

JSONsecurity = '+
{"client_id":"Rest-API-xxxxxxxxxxxxxxxxx",+
"client_secret":"xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx",+
"username":"xxxxxxxxxxxxxx",+
"password":"xxxxxxxxxx",+
"scope":"calc-rest-api",+
"grant_type":"password"}';

URL = 'https://auth.vertexsmb.com/identity/connect/token';

monitor;
resp= http_string('POST': url : JSONsecurity : 'x-www-form-urlencoded');
on-error;
httpcode = http_error();
endmon;
*inlr = *on;


Results from different "Content-Type" below
// 'x-www-form-urlencoded' 400 Bad Request (this was successful in Postman)
// 'application/json' 415 Unsupported Media Type
// 'accept: text/json' 400 Bad Request
// 'text/json' 415 Unsupported Media Type
// 'text/plain' 415 Unsupported Media Type
// 'accept: text/plain' 400 Bad Request

Thanks
Ron