Hi Nitisha,
Once you have done your basic registration, you can generate your Registration Unique Code which becomes your candidate id during login. All these login details, the unique codeand password are sent to your registered email id and phone number. 
So if you have forgotten your unique id you can always recover it from your email inbox.
Else if you have deleted that message as well, the when you open the login page of the official website of AIIMS, you can see beside login button, there is an option called "Forgot Your Login details?" Click on that and try to recover your id , by entering your DOB and email id. Then your unique id and password will be sent to your email id.

If none of the above works, don't worry, AIIMS will start regeneration of Unique Code from 23rd February and continue till 12th March, 2019 5.00pm. So you can regenerate your code then and proceed further for final registration.

Hope this helps.
Comment below for any more queries.

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