Transparent proxy might be what you are looking for (e.g. use proxy for selected traffic without specifying it at the client)
After you setup the proxy , redirect all passing traffic that should be proxied to the proxy port.
Here the MT has IP address 192.168.87.1 and all port 80 traffic should be proxied except that from or to the MT router.
"!" means NOT
# create proxy with the features you like
/ip proxy
set enabled=yes max-cache-size=3000KiB parent-proxy=0.0.0.0
#
# now redirect traffic with dst-port=80 via proxy port 8080, exclude router traffic
/ip firewall nat
add action=redirect chain=dstnat comment="transparent proxy" dst-address=!192.168.87.1 dst-port=80 protocol=tcp src-address=!192.168.87.1 to-ports=8080
#