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Running HAproxy on Proxmox

I installed Proxmox and I want to make the 2nd scheme in the picture.

For this, I installed HAproxy (1.7.8) on Proxmox (5.4) and edited the HAproxy’s config file. I get a 503 error when I try to access it with domains. What could be the reason for this?

If the second scheme is not possible, can I at least make the first scheme?