JSFiddle - React, Tailwind, and code Playground

HTML

<select id="someSelect"></select>

<span id="product_kleuren" class="extra_fields_value">Naturel, Helder, Zwart</span>

JavaScript

var kleuren = document.getElementById('product_kleuren');
    var parts = kleuren.innerHTML.split(",");

    for (var i = 0; i < parts.length; i++) {
        var option = document.createElement("option");
        option.text = parts[i];
        option.value = i;
        document.getElementById("someSelect").appendChild(option);
    }