Create a post with REST API and adding a category

Solution was simple. Set DOM element value in JS variable instead of “document.getElementById(‘post_category’).value” in array.

var submitBtn = document.getElementById('submit-post-btn');

submitBtn.addEventListener('click', function(){
  var categoryTerm = document.getElementById('post_category').value;
  var postData = {
  "title": document.getElementById('post_title').value,
  "content": document.getElementById('post_content').value,
  "status": 'publish',
  "categories": [categoryTerm]
  }
};