Check if the URL already contains a query argument and choose whether to output a '?' or '&' character before any other string.

query-separator-smart-tag.js

function mySep() {
    var url = window.location.href;

    if (url.indexOf('?') != -1) {
        var sep = '&';
    } else var sep = '?';

    console.log(sep);
    return sep;
}

mySep();

const addSep = mySep();
// custom smart tag in OM
document.addEventListener('om.Dtr.init', function (event) {
    event.detail.Dtr.setCustomVariable('separator', addSep);
});