As per the rules of the examination, you can get your registration number by trying this way.

When you have registered you should have received your application number/registration number in your Phone number/email address. So search it out in your old emails/old messages section.

For FYJC CET visit the link below:

https://news.careers360.com/maharashtra-11th-admission-2021-cet-dates-for-fyjc-admission-be-announced-after-ssc-result

I hope this helps.