your question is not clear. the sample app uses a service account (clientid & secret), and does not support login. it just a demo of common graphapi calls
if you are using react, then you probably want to use msal.js for login:
https://learn.microsoft.com/en-us/azure/active-directory/develop/msal-b2c-overview
there is also a react wrapper:
https://learn.microsoft.com/en-us/azure/active-directory-b2c/enable-authentication-react-spa-app