Depends a lot on your existing reverse proxy.
You can read the nginx config that the defaults include and it’s some basic rules to route incoming requests to either lemmy or lemmy-ui. If your existing reverse proxy is nginx you could just incorporate the rules in there.
It also depends on why you need it behind the existing proxy, and how you’ll choose to route your traffic, and where you traffic is coming from in general.
I’d start with taking a look at the default nginx config to see if you can move those rules to your existing reverse proxy, or just forward everything coming in that’s for lemmy straight to the lemmy reverse proxy, although that might be more complicated in correctly preserving the incoming requests.
“For example, if you send a message from Beeper to a friend on WhatsApp, the message is encrypted on your Beeper client, sent to the Beeper web service, which decrypts and re-encrypts the message with WhatsApp’s proprietary encryption protocol.”
So, not really end to end for most common use-cases.