Here we go. Please bear in mind that this tool is not for the general public, and you use it at your own risk.
Below is an example for a fictional multi-city trip for 1 pax: GVA-AMS-SLC-LGB (on KL codes) on Sep 11 and FLL-JFK-ZRH (on DL codes) on Sep 19.
In the URL, each line is a flight and you have to fill the fields with the origin, date, time, flight number. You can have up to 3 flights per direction (I have 3 and 2 in my example).
You can replace X by the fare class and ZZZ by the fare name, but it's not mandatory. AFKL will let you select your fare on their website.
Then, paste this in your brower (as a single line)... and enjoy the ride!*
I find it useful to optimize my transfers (e.g. make them longer) or force an unusual routing. Occasionnaly it has helped me see possibilities even the call center could not see, when I enquired about a flight change.
*The example below has been slightly edited to be displayed properly. You'll have to replace dL by DL twice, if you want it to work.