You can find the shortcode for the checkout button when listing Paddle products or subscription plans on your WordPress dashboard.
[paddlepress product_id="640326" label="Buy Now!"]
This shortcode will render a standard checkout button. However, if you want to use more stylish buttons (e.g custom buttons generated with page builders). You can use them by just adding some attributes.
The class name “paddle_button” is an identifier for the Paddle.js library. As long as you pass the correct data attributes with the correct class name, you can initialize overlay checkout everywhere on your website.
<a href="#!" class="paddle_button" data-product="12345" data-theme="none">Buy Now!</a>
If you need more detailed guide for that, checkout Paddle developer guide here.
In order to use Paddle checkout successfully, your domains has to be approved. Read more.