I looked up the solutions that you provided. Thank you again 
The post-user-registration doesn’t work, because I couldn’t send POST Request to my localhost local server due to the localhost refers to the server instance the Actions/Rules/Hooks are running. It is also not very secure I guess. Refer to: Is there a way to use "localhost" as the axios post destination for Auth0 Actions?
I also couldn’t fully understand how I can get the User ID via silent authentication How exactly I can do it?
If the both ways won’t work, Do you know how can I send the user ID right after the user authentication(sign up) on my client side? I think I can use the callback url after user authenticates but it also redirects the callback url after the login process too. So, the User ID is sent at every user login and I don’t desire that.
I’m not sure but I could not find any best practice solution in the documentation for this problem even though it is a common case for applications?
Thank you for your help again!