If the PC Client behind the MiktortikOS want to visit http://www.xyz.com the MIKROTIK will redirect to http://www.abc.com.
How to do it?

you have to use transparent proxy for the clients, and then use the proxy access list to do redirects as you want.

Enable the web proxy, you don't need modify nothing in the web proxy then go to firewall and do the next step:

1.- Into the NAT section add
chain: dst-nat
protocol: tcp
dst-port: 80
action= redirect
to-port:8080

2.-Back to webproxy panel and add the rule.