StateToken / POST Request to Apps Script Function - google-apps-script

Question

I am building a Woocommerce API integration and am working on the authentication functionality. The authorization endpoint requires a return_url and a callback_url.
I am under the impression that I can use the StateTokenBuilder, similar to how the OAuth2 library functions but am running into issues trying to implement based on the Apps Script docs.
Of course, the UrlFetchApp to the Woocommerce endpoint works fine, but when trying to authorize the app at the endpoint, I get the following error:
Error: An error occurred in the request and at the time were unable to
send the consumer data.
After subsequent tests with Postman, the callback URL, and return URLs evaluate to the Google Drive splash page and do not execute the function in the script. I am at a loss for how to access the callback function from another source, as it appears that it is only accessible via browser, when logged in.
Below are my functions:
let endpoint = {
protocol: 'https',
base: '{URL}.com'
}
let params = {}
API.Auth.Authorize = options => {
endpoint = { ...endpoint, ...{ section: 'wc-auth/v1/authorize' } };
options = { ...params, ...options };
let method = 'GET';
const query =
method === 'GET' && typeof params === 'object'
? Object.keys(options)
.filter(param => param !== 'payload')
.map(param => `${param}=${encodeURIComponent(options[param])}`)
.join('&')
: options;
const url = `${endpoint.protocol}://${endpoint.base}/${endpoint.section}${typeof query === 'string' && query.length > 0 ? `?${query}` : ''}`;
let opts = {
method: method,
muteHttpExceptions: true
};
return url;
};
const authorize = () => {
const params = {
app_name: 'App Name',
scope: 'read_write',
user_id: 'abc123',
return_url: 'https://script.google.com/macros/s/{SCRIPT_ID}/exec',
callback_url: generateCallbackURL('handleCallback')
};
const result = API.Auth.Authorize(params);
console.log(result);
}
generateCallbackURL = fn => {
const script_url = `https://script.google.com/macros/d/${ScriptApp.getScriptId()}`;
const url_suffix = '/usercallback?state=';
const state_token = ScriptApp.newStateToken().withMethod(fn).withTimeout(120).createToken();
const callback_url = `${script_url}${url_suffix}${state_token}`;
return callback_url;
}
const handleCallback = e => {
return HtmlService.createHtmlOutput(JSON.stringify(e));
}
I have also tried replacing the handleCallback function with a doPost function to determine if that would solve the issue. It did not have an effect.