for port 80
eth1=LAN Interface
192.168.2.1 = IP of Squid Proxy Server LAN interface (eth1)
192.168.100.0/24 = Network for LAN users
192.168.2.254 = IP Mikrotik Proxy Interface IP (Squid)

/interface ethernet
set 0 name=LAN
set 1 name=SQUID
set 2 name=WAN

/ip address
add address=192.168.1.1/24 interface=WAN network=192.168.0.0
add address=192.168.2.254/24 interface=SQUID network=192.168.50.0
add address=192.168.100.254/24 interface=LAN network=192.168.100.0


/ip firewall mangle
add action=mark-routing chain=prerouting dst-port=80 new-routing-mark=http protocol=tcp
/ip firewall nat
add chain=srcnat dst-port=80 protocol=tcp

/ip route
add distance=1 gateway=192.168.2.1 routing-mark=http


+ port 443 "https"

/ip firewall mangle
add action=mark-routing chain=prerouting dst-port=443 new-routing-mark=https protocol=tcp
/ip firewall nat
add chain=srcnat dst-port=443 protocol=tcp
/ip route
add distance=1 gateway=192.168.2.1 routing-mark=https