TLDR - The URL scheme is upi://pay?pn=<Name>&pa=<UPI ID>&cu=<Currency Code>
All UPI apps support this custom URL scheme, but many websites don’t seem to use it. When you click this type of link, the “Open With” menu will appear with a list of all the UPI applications on your phone. If you are on a mobile device, you can try this link: upi://pay?pn=Vikas%20Kumar&pa=vikaskr@freecharge&cu=INR. It should redirect you to a UPI app.
Although this link will not work on your computer, you can use a media query to replace it with a QR code. Alternatively, you can use JavaScript to detect whether the URL scheme is supported, but in my tests, that approach did not seem reliable.