Integrating PayPal's Express Checkout into your product landing pages requires a Client ID that authenticates a merchant's account with PayPal. A Client ID can only be created by the PayPal account holder.
How to get a Client ID
1. Login to the PayPal developer site
2. Create an app
Click your name in the upper-right corner, then click Dashboard. You will be given the Create App option to create your new application. Fill in the details for the new app and after confirmation you will be presented with the Client ID for the sandbox on the next page.
The Sandbox Client ID is for testing purposes and does not make real payments.
To get a client ID for productive use, switch to the applications.liveToggle tab in the dashboard and follow the instructions.