How to Add Custom Field Options Alongside Post Add Card?

Hi! I’m fairly to the Trello API community so sorry if this question is silly. I was wondering how to add options to a pre-defined custom field (Text custom field) with a post request that also creates a card.

I got the post request to create a card, and it would be really awesome if I knew how to add to the custom field options that is unique for each created card; based on what I set it in the post reqeust.

function createTrelloCard(items) {
   var data = { "value": { "text": "Hello, world!" } }
   var payload = {"name":items,
                  "due": "", 
                  "customField" : data, // I attempt custom field here.
                  //"labels": ,
                  //"idMembers": ,
                  //"idCardSource": ,
   var url = '[APPKEY]&token=[TOKEN]' //optional... -&cards=open&lists=open'-
   var options = {"method" : "post",
                  "payload" : payload};

   UrlFetchApp.fetch(url, options);

Not a silly question at all!

There isn’t a way to set the value for a custom field on a card at the time of card creation. You can only update custom field data in a separate API call after the card has been created. This is similar to how checklists and attachments work, as well. They can only be added to a card after the card has been created.