How to connect Mikrotik to an external proxy server

Hi,

I have an external http proxy with user name and pass

How can I make mikrotik router to direct all clients traffic to be connected to this proxy server. I have
Proxy ip
Proxy port
Username
Pass

You could use a DHCP option.

if you don’t mind. could you tell me how to make it?

This should answer your questions.
http://forum.mikrotik.com/t/set-web-proxy-address-in-browser-automatically-like-wpad/50485/1