Auth API calls resulting in 400 bad requests when POSTing multipart/form-data payloads with SSJS

Question

I'm trying to make Auth API calls to an external API Endpoint from Marketing Cloud using SSJS or AMPScript but I am getting 400 bad request errors for these calls. In talks with a contact from the API endpoint, the endpoint is expecting the calls to have a Content-type of "multipart/form-data" whereas most examples online are utilizing 'application/json' type.

I've tried it with AMPScript and SSJS in a Cloudpage, Code Resource, and Email to attempt to debug the code but we continue to get 400 bad request errors.

I attempted to form the payload utilizing the example outlined in this post but the call still resulted in a 400 error.

These Auth calls are successful in postman but we are passing the grant_type and Scope in the "form-data" section of call(s). I tried to pull out the Javascript from Postman and use it in an SSJS code snippet but I didn't have any luck.

Here is my SSJS code with security information redacted.

<script runat="server">
Platform.Load('core', '1.1');
    
var url = '*****REDACTED*****';
      
var boundary = 'Test';
      
var headerNames = ['Customheader', 'Cache-Control'];
    
var headerValues = ['X', 'no-cache'];
    
var contentType = 'multipart/form-data; charset=utf-8; boundary=' + boundary
    
var result = [0];
    
var payload = '';
    payload += '\r\n--' + boundary
    payload += '\r\nContent-Disposition: form-data; grant_type="client_credentials"; scope="****REDACTED****"';
    payload += '\r\n\r\nclient_id,client_secret'
    payload += '\r\n****REDACTED*******,****REDACTED*******'
    payload += '\r\n--' + boundary + '--'
    // Write('Payload: ' + payload)

try
{
    var statusCode = Platform.Function.HTTPPost(url, contentType, payload, headerNames, headerValues, result);
}catch(error) {
    Write(error.description)
}
  
</script>

Here is the error message from an Email preview:

ExactTarget.OMM.FunctionExecutionException: An error occurred when attempting to evaluate a HTTPPost function call. See inner exception for details. Error Code: OMM_FUNC_EXEC_ERROR - from Jint --> --- inner exception 1--- System.Net.WebException: The remote server returned an error: (400) Bad Request. - from System

Here is another example where we stringify the payload.

    <script runat="server">
Platform.Load('Core', '1.1')  
    var client_id = '*REDACTED*';
    var client_secret = '*REDACTED*';
    var scope = "*REDACTED*"
      var encode = Base64Encode(client_id + ':' + client_secret);
      var authHeader = 'Basic ' + encode;
  var url = '*REDACTED*';
var contentType = 'multipart/form-data';
  var payload = 'grant_type=client_credentials&scope=*REDACTED*';
  var strpayload = Stringify(payload);
  var headerNames = ["Authorization"];
var headerValues = [authHeader];

 var result = HTTP.Post(url, contentType, strpayload, headerNames, headerValues);
   
</script>

Any help getting these Auth calls up and running would be much appreciated.