I want to integrate Mpesa API to a google form. Users using the form to pay, let us say for registration Fee
You now use M-Pesa RESTful APIs accessible through Safaricom's developer portal. The documentation has sample codes in the major programming languages - Python, JavaScript/NodeJS, Java, Ruby.
You can now consume the M-Pesa RESTful APIs with Google Apps Script like you would any other RESTful APIs.