How can I report to an ID for plagiarism ?

I want to report someone who copied and submitted codes in August long challenge. I can show you the proof also from where he copied those codes.
I want to report him to admin or moderator coz I didn’t want to post his name publicly.

You can use the following (public) post: Updates on curtailing plagiarism

A private way would be using the email an admin linked in the above post:

We request everyone to report plagiarism cases on within 3 days of each contest.

Also note that you should probably not report cheatings from last year August.