To mark traffic based on website I had to create rules to first identify the website then put the address of the website into an address list to then mark the traffic. This had to be done two different ways, one for regular traffic (HTTP) and one for secure traffic (HTTPS). To mark the HTTP traffic I used the content argument under the advanced tab.
/ip firewall mangle add action=add-dst-to-address-list address-list=Video address-list-timeout=0s \ chain=prerouting comment="Mark BBC Address" content=bbc.co.uk disabled=no \ dst-port=80 protocol=tcp
Then to mark the secure traffic you have to use Layer7 Protocols. To do this add an expression with the domain you are wanting to catch.