Convert Product Addon Field to Select2

product-addons.php

<!-- Add following snippet to "Header & Footers WordPress Plugin. Replace wc.local with your own website URL -->

<link rel='stylesheet' id='select2-css'  href='https://wc.local/wp-content/plugins/woocommerce/assets/css/select2.css?ver=4.6.1' media='all' />
<script src="https://wc.local/wp-content/plugins/woocommerce/assets/js/selectWoo/selectWoo.full.min.js?ver=1.0.6" id="selectWoo-js"></script>

<script>
jQuery( document ).ready(
	function ($) {
		$( '.wc-pao-addon-select' ).filter( ':not(.enhanced)' ).each(
			function() {
				jQuery( this ).select2(
					{
						width: '100%',
					}
				).addClass( 'enhanced' );
			}
		);
	}
);
</script>

<style>

.select2.select2-container {
  width: 100% !important;
}

.select2.select2-container .select2-selection {
  border: 1px solid #ccc;
  -webkit-border-radius: 3px;
  -moz-border-radius: 3px;
  border-radius: 3px;
  height: 34px;
  margin-bottom: 15px;
  outline: none;
  transition: all 0.15s ease-in-out;
}

.select2.select2-container .select2-selection .select2-selection__rendered {
  color: #333;
  line-height: 32px;
  padding-right: 33px;
}

.select2.select2-container .select2-selection .select2-selection__arrow {
  background: #f8f8f8;
  border-left: 1px solid #ccc;
  -webkit-border-radius: 0 3px 3px 0;
  -moz-border-radius: 0 3px 3px 0;
  border-radius: 0 3px 3px 0;
  height: 32px;
  width: 33px;
}

.select2.select2-container.select2-container--open .select2-selection.select2-selection--single {
  background: #f8f8f8;
}

.select2.select2-container.select2-container--open .select2-selection.select2-selection--single .select2-selection__arrow {
  -webkit-border-radius: 0 3px 0 0;
  -moz-border-radius: 0 3px 0 0;
  border-radius: 0 3px 0 0;
}

.select2.select2-container.select2-container--open .select2-selection.select2-selection--multiple {
  border: 1px solid #34495e;
}

.select2.select2-container.select2-container--focus .select2-selection {
  border: 1px solid #34495e;
}

.select2.select2-container .select2-selection--multiple {
  height: auto;
  min-height: 34px;
}

.select2.select2-container .select2-selection--multiple .select2-search--inline .select2-search__field {
  margin-top: 0;
  height: 32px;
}

.select2.select2-container .select2-selection--multiple .select2-selection__rendered {
  display: block;
  padding: 0 4px;
  line-height: 29px;
}

.select2.select2-container .select2-selection--multiple .select2-selection__choice {
  background-color: #f8f8f8;
  border: 1px solid #ccc;
  -webkit-border-radius: 3px;
  -moz-border-radius: 3px;
  border-radius: 3px;
  margin: 4px 4px 0 0;
  padding: 0 6px 0 22px;
  height: 24px;
  line-height: 24px;
  font-size: 12px;
  position: relative;
}

.select2.select2-container .select2-selection--multiple .select2-selection__choice .select2-selection__choice__remove {
  position: absolute;
  top: 0;
  left: 0;
  height: 22px;
  width: 22px;
  margin: 0;
  text-align: center;
  color: #e74c3c;
  font-weight: bold;
  font-size: 16px;
}

.select2-container .select2-dropdown {
  background: transparent;
  border: none;
  margin-top: -5px;
}

.select2-container .select2-dropdown .select2-search {
  padding: 0;
}

.select2-container .select2-dropdown .select2-search input {
  outline: none;
  border: 1px solid #34495e;
  border-bottom: none;
  padding: 4px 6px;
}

.select2-container .select2-dropdown .select2-results {
  padding: 0;
}

.select2-container .select2-dropdown .select2-results ul {
  background: #fff;
  border: 1px solid #34495e;
}

.select2-container .select2-dropdown .select2-results ul .select2-results__option--highlighted[aria-selected] {
  background-color: #3498db;
}
</style>