You must have PayPal Business account to be able to connect payment to your website. You can upgrade your standard account to business in your PayPal dashboard.
Go to https://developer.paypal.com/developer/applications/ and log in with your PayPal business account credentials
Navigate to the My Apps & Credentials tab and
click on Live button
Click the Create App button in the REST API apps section
Enter your shop name in App Name
Click the Create App button
The App will be created and you will see Client ID on the page.
Copy Client ID key in API CREDENTIALS tab.
Paste this key in Client ID field in mssg.me service.
Tip: Use Sandbox Client ID to test integration in mssg.me service.