JSFiddle - React, Tailwind, and code Playground

HTML

<select id="categories" name="categories[]" multiple="multiple" onchange="sendCategories(this);">
    <option value="asdf">asdf</option>
    <option value="fdsa">fdsa</option>
    <option value="fff">fff</option>
    <option value="aaa">aaa</option>
</select>

JavaScript

function sendCategories(sel) {
    var values = [], i, j, cur;
    
    for (i = 0, j = sel.options.length; i < j; i++) {
        cur = sel.options[i];
        if (cur.selected) {
            values.push(encodeURIComponent(cur.value));
        }
    }
    
    if (values.length) {
        values = encodeURIComponent(sel.name) + "=" + values.join("&" + encodeURIComponent(sel.name) + "=");
    } else {
        values = null;
    }
    
    alert(values);
}