In main.js you can see how to initialize the widget and how to handle the responses. To test, edit main.js and replace the following lines with your own values.
const apiKey = '{your-api-key}';
const userId = '{your-user-id}';
const vendorId = '{your-vendor-id}';
apiKey and vendorId are supplied by GrailPay. userId is the UUID of a user you have created using the register/person API endpoint.