tshark command to add a non-default port to HTTP

From the tshark man page:

-o <preference>:<value>    
    Set a preference value, overriding the default value and any value read from a preference file. The argument to the option is a string of the form prefname:value, where prefname is the name of the preference (which is the same name that would appear in the preference file), and value is the value to which it should be set.

So, in your case you would use -o http.tcp.port:80,8080,nnnn where nnnn is your required port and you also list all other required http ports as the preference setting is a "range.".

Tip: The preference "name" is shown as a tooltip in the GUI when you hover over it (for protocol prefs).