Step 1: Get Application Credentials
Sign in to the Square Developer Dashboard.
Create a new Square application, and then name it In-App Payments SDK Quick Start.
Choose the In-App Payments SDK Quick Start application to open it.
Toggle Production to Sandbox to show Sandbox credentials.
Open the Credentials page, and then copy your Sandbox Application ID and Sandbox Access Token.
Open the Locations page, and then copy the Default Test Account location ID.