you need:
an external disk on the USB port
the containers plugin on RouterOS7

There are several ways to connect the HA-Proxy container to the network, here we chose to give it its own IP.
follow the instructions to create a virtual network interface with IP and connection to the desired VLAN.

For the persistent volume of the container we create a directory

/file 
add type=directory name=/usb1/persistent-volumes/haproxy

Start by creating the config file on your workstation, vi haproxy.cfg, add the following content for a test to the website at 10.100.10.103

global
    daemon
    maxconn 256

defaults
    mode http
    timeout connect 5s
    timeout client  50s
    timeout server  50s

frontend http_in
    bind *:80
    default_backend web_servers

backend web_servers
    balance roundrobin
    server srv1 10.100.10.103:80 check