When you made successful registration of KCET 2020 you must have received an e-mail from the respective authorities. That e-mail includes all your login credentials with your User ID.
You can check for it in your mail inbox or it might be in your spam folder.
I hop this information helps you.
For further queries feel free to ask in comment section.
If you have forgotten your user ID you can check your registered email and phone number as your details will be sent to you when you have completed your registration. You can also retrieve your user ID by sending a mail to email@example.com or call HelplineNumber: 080 - 23 460 460