JSFiddle - React, Tailwind, and code Playground

HTML

<select id="idselect" class="ui-corner-all ui-widget-content" multiple="multiple" name="idPosto">
    <option value="1">Opção 1</option>
    <option value="2">Opção 2</option>
    <option value="3">Opção 3</option>
    <option value="4">Opção 4</option>
</select>
<p><a id="selectTarget" href="selecionados.php?precisoqueconcateneaqui">Link</a>
</p>

CSS

#idselect {
    padding: 3px 4px;
}

JavaScript

function changeHref(e) {
    var options = [].map.call(this.selectedOptions, function (el) {
        return el;
    });
    var root = link.href.split('?')[0];
    var queryString = options.map(function (opt) {
        return 'idselect[]=' + opt.value;
    }).join('&');
    link.href = [root, '?', queryString].join('');
}

var select = document.getElementById('idselect');
var link = document.getElementById('selectTarget');
select.addEventListener('click', changeHref);