Hello Parimala,
After doing your registration, you can generate the Registration Unique Code in your profile which is your candidate id during login. Once you complete all the formalities of registration all these login details, the unique code and password are sent to your corresponding email id and phone number. 
So if you have forgotten your unique code you can always recover it from your email inbox.
Else if you have deleted that message as well, then 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 and code by entering your DOB and email id. Then your unique id and password will be sent to your email id.

Comment below if you still face an issue.

Hello aspirant,

If you have lost your registration code, you may check your email as they mail all your details, you will surely find it there or they also message the information, you must have the back up of previous messages you may also find it there. Hope you get the registration code by the help of information provided. 

Best of luck!!