User ID is an identity provided by kintone.com system.
It is created when the user is added. User IDs cannot be changed or deleted.
You can find the user ID in the URL shown on "Edit User" screen.
Click in the header.
Click "Users & System Administration".
Accessing Kintone Users & System Administration
Click Departments & Users.
Enter a display name or a login name of the user in the search box to search the target user.
You can also expand the department tree and select the target user.
Click for the user.
Check the "id = (number)" at the end of the URL.