JSFiddle - React, Tailwind, and code Playground

HTML

<script src="//app-sji.marketo.com/js/forms2/js/forms2.min.js"></script>
<form id="mktoForm_96"></form>
<form id="confirmform"><input type="text" value="I am in the confirm form"></form>
<script>
    MktoForms2.loadForm("//app-sji.marketo.com", "848-DID-242", 3329,
        function(form) {

            //add an onSubmit handler
            form.onSubmit(function() {

                // Hidden `Calculator Quote URL` from current URL
                var currentURL = window.location.href;

                // Hidden `Company` from existing fields
                var company = form.getValues().FirstName + ' ' + form.getValues().LastName;

                // Set hidden fields
                form.addHiddenFields({
                    "Calculator_Quote_URL__c": currentURL,
                    "Company": company,
                    "Lead_Created_Date__c": ""
                });

            });

            form.onSuccess(function() {
                form.getFormElem()[0].style.visibility = 'hidden';
                document.getElementById('confirmform').style.visibility = 'visible';
                return false;
            });

        });
</script>

CSS

#confirmform {
    visibility: hidden;
}