Explanation
The best option for UC to meet the requirements is to add an Apex callout in the registration handler of the authorization provider. An authorization provider is a configuration in Salesforce that allows users to log in with an external authentication provider, such as Facebook. A registration handler is an Apex class that implements the Auth.RegistrationHandler interface and defines the logic for creating or updating a user account when a user logs in with an external authentication provider. An Apex callout is a method that invokes an external web service from Apex code. By adding an Apex callout in the registration handler, UC can create a customer account in their accounting system by calling the web service that is accessible to Salesforce. This option enables UC to automate the account creation process and integrate with their existing accounting system. The other options are not optimal for this scenario. Creating a custom application on Heroku that manages the sign-on process from Facebook would require UC to develop and maintain a separate application and infrastructure, which could increase complexity and cost. Using JIT provisioning to automatically create the account in the accounting system would require UC to configure Facebook as a SAML identity provider, which is not supported by Facebook. Using OAuth JWT flow to pass the data from Salesforce to the accounting system would require UC to obtain an OAuth token from the accounting system and use it to make API calls, which could introduce security and performance issues. References: [Authorization Providers],
[Create a Registration Handler Class], [Auth.RegistrationHandler Interface], [Apex Callouts], [Facebook as SAML Identity Provider], [OAuth 2.0 JWT Bearer Flow for Server-to-Server Integration]