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/config-dir

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

upload this config file to the preveously crated location.

scp haproxy.cfg styx:/usb1/persistent-volumes/haproxy/config-dir

Now set the volume with the config file as a persistent volume mount in the container:

/container/mounts
add name=HAPROXY_CONF dst=/usr/local/etc/haproxy src=/usb1/persistent-volumes/haproxy/config-dir

/container/add remote-image=haproxy:latest interface=veth-haproxy root-dir=usb1/persistent-volumes/haproxy/root-dir mounts=haproxy name=haproxy
/container/start haproxy