Thanks to KG(VMWARE) (not sure if I can mention his full name here) for the help. As he mentioned I have to pass an argument in the REST call to archive the report.
Initially I was doing the following which do not archive the reports.
https://CBHostName:443/vCenter-CB/api/report + all the POST xml input as in this post
Now I am doing the following which solved the problem.
https://CBHostName:443/vCenter-CB/api/report?autoArchive=true + all the POST xml input as in this post