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