Restrict Custom field duplicate data on Power Portal Registration

In Portal registration form I need to add two custom fields

1. Date of Birth
2. SSN Number

Note :- I have added two fields using content Snippet also added fields on contact database.

Now, I wanted to restrict users to create duplicate record of Date of Birth and SSN Number. If user trying to add SSN and date of birth which is already exists in database, then to show message pop up “User already register”

I have tried using alternate key applying on two fields but, I don’t want to use as it’s direct operation on database side.

Also, can I handle using plugin is yes, how can plugin run on register button of portal page.

Can you please suggest best approach to handle above requirement?