appname.yml

http:
  routers:
    appnamehere-rtr:
      rule: "Host(`appnamehere.yourdomainnamehere.com`)"
      entryPoints:
        - https
      middlewares:
        - chain-authelia
        - appnamehere-add-admin
      service: appnamehere-svc
      tls:
        certResolver: dns-cloudflare
  middlewares:
    appnamehere-add-admin:
      addPrefix:
        prefix: "/admin"
  services:
    pihole1-svc:
      loadBalancer:
        servers:
          - url: "http://appipaddresshere:appporthere"

you can also remove the adding of prefixes like /admin by doing this instead

http:
  routers:
    appnamehere-rtr:
      rule: "Host(`appnamehere.yourdomainnamehere.com`)"
      entryPoints:
        - https
      middlewares:
        - chain-authelia
      service: appnamehere-svc
      tls:
        certResolver: dns-cloudflare
  services:
    reofront-svc:
      loadBalancer:
        servers:
          - url: "http://appipaddresshere:appporthere"