Trying to proxy Pihole admin but getting 502s and 404s

I'm essentially trying to proxy my pihole admin panel so that I can reach it by going to something like "" and it show up there so that I don't have to rely on port 80.

Here is my config:
This is the command I'm running and the error I'm getting:

I feel like I'm very close, but can't quite figure out what is going wrong here. Can someone please help point me in the right direction?