get_company_list.html

<link href="https://cdnjs.cloudflare.com/ajax/libs/select2/4.0.3/css/select2.min.css" rel="stylesheet" />
<script src="https://cdnjs.cloudflare.com/ajax/libs/select2/4.0.3/js/select2.min.js"></script>

<script>
var company_field = jQuery('#acf-field-company_name');

var is_admin = jQuery('#post_author_override')[0];

if(is_admin != undefined){
	company_field.select2({

		ajax: {
			url: 'http://localhost:8000/get_user_companies/',
			processResults: function (data) {
				var arr = []
				jQuery.map(data.companies, function(val, i){
					arr.push({id:val.slug, text: val.name})
				});
				return {
					results: arr
				};
			},
			xhrFields: {
				withCredentials: true
			},
			method: 'POST',
			dataType: 'json',
		}

	});

}

else{

	jQuery.ajax({
		url: 'http://localhost:8000/get_user_companies/',
		method: 'POST',
		dataType: 'json',
		xhrFields: {
	    withCredentials: true
	  }
	})
	.done(function (result) {
		data = result.companies
		jQuery.map(data, function (val, i) {
			company_field.append('<option value='+ val.slug +'>'+ val.name +'</option>');
		});
	});

	company_field.select2();

}

var post_id = jQuery('#post_ID').val();

if (post_id){
	jQuery.ajax({
		url: 'http://localhost:8000/get_post_companies/',
		method: 'POST',
		data: {'post_id': post_id},
		dataType: 'json',
		xhrFields: {
			withCredentials: true
		}
	})
	.done(function (result) {
		data = result.companies
		jQuery.map(data, function (val, i) {
			company_field.append('<option value='+ val.slug +' selected>'+ val.name +'</option>');
		});
	});

}

</script>