To open a desktop application using a website button click event, you can utilize custom URL schemes or protocol handlers. Here’s how you can achieve this:

1. Determine the custom URL scheme or protocol handler for the desktop application you want to open. Many desktop applications register their own custom URL scheme or protocol handler to allow external applications or websites to interact with them.
2. Create a button or link on your website that triggers the application launch. This button should have an onclick event handler that opens a new window or tab with the custom URL scheme or protocol handler.

Here’s an example using JavaScript:

<button onclick="openDesktopApplication()">Open Desktop Application</button>

<script>
function openDesktopApplication() {
  // Replace "appname://" with the custom URL scheme or protocol handler for your desktop application
  window.open("appname://", "_blank");
}
</script>

In this example, when the button is clicked, the openDesktopApplication() function is triggered, which opens a new window or tab with the custom URL scheme or protocol handler. Make sure to replace "appname://" with the appropriate custom URL scheme or protocol handler for the desktop application you want to open.

Note that the availability and behavior of custom URL schemes or protocol handlers depend on the specific desktop application and the user’s operating system configuration. Some applications may require additional parameters or data to be passed through the URL to perform specific actions.

It’s important to mention that the user’s device should have the desktop application installed and properly configured with the custom URL scheme or protocol handler for this method to work.