Pre-populate Mautic Form Data from Query String Parameters

mautic-form-preload.js

document.onreadystatechange = function () {
    if (document.readyState == 'interactive') {
        if (document.forms.length !== 0 && location.search) {
            var query = location.search.substr(1);
            query.split('&').forEach(function (part) {
                if (part.indexOf('=') !== -1) {
                    var item = part.split('=');
                    var key = item[0];
                    var value = decodeURIComponent(item[1]);
                    var inputs = document.getElementsByName('mauticform[' + key + ']');
                    inputs.forEach(function (input) {
                        input.value = value;
                    });
                }
            });
        }
    }
}

@shawncarr I forked your script to work if run after the form loads and to handle checkboxes: https://gist.github.com/stevedrobinson/66aa33f3f26ee81e80ae49fd89390713

Thought you might like to update yours, since I'm not sure how to do PRs on gists :)