index.html

<form>
    <script src="https://api.payant.ng/assets/js/inline.min.js"></script>
    <button type="button" onclick="payWithPayant()"> Pay </button> 
</form>

<script>
    function payWithPayant() {
        var handler = Payant.invoice({
            "key": "PUBLIC-KEY",
            "client": {
                "first_name": "Albert",
                "last_name": "Jane",
                "email": "albert.jane@domain.com",
                "phone": "+2348012345678"
            },
            "due_date": "12/30/2019",
            "fee_bearer": "client",
            "items": [
                {
                    "item": "Donuts",
                    "description": "The best donuts in the world!",
                    "unit_cost": "1000.00",
                    "quantity": "1"
                }
            ],
            callback: function(response) {
                console.log(response);
            },
            onClose: function() {
                console.log('Window Closed.');
            }
        });

        handler.openIframe();
    }
</script>