Код:

<input type="button" value="Оплатить"  onclick="makePayment()">

<script type="text/javascript">
     function makePayment() {
        var WidgetRequestData = {
            "LMI_MERCHANT_ID":"1412",
            "LMI_PAYMENT_NO":"o-2193987",
            "LMI_PAYMENT_AMOUNT":"10.99",
            "LMI_PAYMENT_DESC":"Order description"
        };

        PMWidget
                .init({
                    mode: 'popup',
                    params: WidgetRequestData,
                    closeOnFail: true
                })
                .on('ready', function() {
                    console.log('READY');
                })
                .on('success', function() {
                    if('undefined' !== typeof WidgetRequestData.LMI_SUCCESS_URL) {
                        window.location = WidgetRequestData.LMI_SUCCESS_URL;
                    }
                })
                .on('fail', function(params) {
                    console.log('FAIL ' + params.message);
                })
                .on('close', function() {
                    console.log('CLOSE');
                })
                .on('error', function() {
                    console.log('ERROR');
                })
        ;
    };
</script>
<script type="text/javascript" src="https://lmi.paysoft.solutions/js/widget/initScript.js" async></script>