You can contact the board's office with a written email or you can check your previous emails in promotion section or spam folder, because during registration the username and password are sent to your respective email that you have provided to the examination board.

You can also check your previous messages on mobile related to KCET because during registration your used id are usually sent on your registered phone number.

Dear student

You can check your SMS from K-CET on the date of registration, as K-CET sends SMS to candidates soon after their registration . The SMS contains User ID and password. If that doesn't work then try checking your email inbox for the mail sent by K-CET as it may also contain the User ID and password. Else send a mail to K-CET mentioning that you forgot your User ID. they will help you.

I hope my answer helped you