php landing page integration with magento 2 using order API

I want to integrate my landing page integration with the magento2 . landing page will have only 1 product and when a customer click on buy now button then he will be redirected to the checkout page and place order after filling the form. The order will go to my magento store usign API (However I have integrated it using payment method cashondelivery). I want someone to integrate razorpay payment method in existing coding or he can do cashondelivery and razorpay from start.