Create a Custom Send as in Gmail using Apps Script

Question

I do apologise for asking this here as there is already an existing thread but I was unsure of the correct protocol.

I need to use apps script to add a custom sendas address for my users in Gmail. The thread here appears to do what I need: Creates a custom "from" send-as alias with GAS and APIs.

However I am new to Apps Script (especially advanced APIs) and am unsure which sections of Jay's script that I would need to update to make this work for me.

I know that I will need to update:

function createAlias() {
       var userEmail = 'useraccount@example.com';
       var alias = 'myalias@example.com';
       var alias_name = 'Alias User';

But I am unsure what to update the following with or where to find it:

var service_account = {
  "private_key": "-----BEGIN PRIVATE KEY...",
  "client_email": "sa-email@example.com",
  "client_id": "1234569343",
  "user_email": "useraccount@example.com"
  };

I was unable to comment on the existing post and it didn't seem appropriate to add my question as an answer. For convenience, I have pasted Jay's code here.

If anyone could please let me know which variables I will need to update with my specific information (and if necessary, where to find it) that would be much appreciated.

Kind Regards,

Brett

var service_account = {
"private_key": "-----BEGIN PRIVATE KEY...",
"client_email": "sa-email@example.com",
"client_id": "1234569343",
"user_email": "useraccount@example.com"
};

function getOAuthService(user) {
  return OAuth2.createService('Service Account')
    .setAuthorizationBaseUrl('https://accounts.google.com/o/oauth2/auth')
    .setTokenUrl('https://accounts.google.com/o/oauth2/token')
    .setPrivateKey(service_account.private_key)
    .setIssuer(service_account.client_email)
    .setSubject(service_account.user_email)
    .setPropertyStore(PropertiesService.getScriptProperties())
    .setScope('https://www.googleapis.com/auth/gmail.settings.sharing 
https://www.googleapis.com/auth/gmail.settings.basic')
}

function createAlias() {
  var userEmail = 'useraccount@example.com';
  var alias = 'myalias@example.com';
  var alias_name = 'Alias User';

  var service = getOAuthService();
  service.reset();
  if (service.hasAccess()) {
  var url = 'https://www.googleapis.com/gmail/v1/users/me/settings/sendAs'
  var headers ={
    "Authorization": 'Bearer ' + service.getAccessToken(),
    "Accept":"application/json", 
    "Content-Type":"application/json",
    };

    var resource ={
      'sendAsEmail': alias,
      'displayName': alias_name
       };

     var options = {
       'headers': headers,
       'method': 'POST',
       'payload': JSON.stringify(resource),
       'muteHttpExceptions': true
       };

       Logger.log(options);
       var response = UrlFetchApp.fetch(url, options);
       Logger.log(response.getContentText());
       }
}

function reset() {
  var service = getOAuthService();
  service.reset();