Domain Mapping

Hello!
I would like to to set up a domain mapping on my site so that I have:

www.productA.com would point to mainsite.com
I want the entire URL to show (both of them) so I don’t want forwarding or making.

How do I go about doing this with Cloudflare enabled? Do I used page rules?

Thanks!

It depends which entire URL you want to show (not sure what both of them means in this instance). But using page rules for the product A domain you would set a 301 for www.productA.com or possibly www.productA.com* forward to mainsite.com.