HAproxy: how to use backend with specific path+port?

[MarvinFS]

Dear colleagues,

Could you please help me, I know the basics: like how to use different certificates with a single front end and different backends with single front end, but I'm stuck with that task:

back ends:
admin 192.168.0.6 port 7071
webapp 192.168.0.6 port 7443

frontend: xxx.xxx.xxx.xxx:443 (public IP)

redirect to backend "admin: if expression: Path contains admin
redirect to backend "webapp" if expression: host matches mail.mydomain.com

So webapp backend which is email web UI, works fine from outside but not admin backend, when I'm trying to access https://mail.mydomain.com/admin
as it strictly requires to be redirected to https://192.168.0.6:7071/ZimbraAdmin/ to be operational
and I can't figure how to specify path AND port number transparently to a client for a backend

I reckon there should be used something like https-request set uri or set path actions in conditions but I can't figure the correct parameters and how to specify it properly- could somebody help me please?

[MarvinFS]

I managed to get redirects working but it redirects to private service address which is obviously not accessible from outside