The account link sequence that should be followed is:
1. The bot server calls the API that issues a link token from the LINE user ID.
2. The LINE Platform returns the link token to the bot server.
3. The bot server calls the Messaging API to send a linking URL to the user.
4. The LINE Platform sends a linking URL to the user. (Fig. 1)
5. The user accesses the linking URL.
6. The web server displays the login screen. (Fig. 2)
7. The user enters their credentials.
8. The web server acquires the user ID from the provider's service and uses that to generate a nonce (number used once).
9. The web server redirects the user to the account-linking endpoint.
10. The user accesses the account-linking endpoint.
11. The LINE Platform sends an event (which includes the LINE user ID and nonce) via webhook to the bot server. (Fig. 3)
12. The bot server uses the nonce to acquire the user ID of the provider's service.