If you do any search using the site you can then inspect the resulting URL to find out how if/how your request is dealt with in the URL.
For http://mxtoolbox.com/blacklists.aspx with example.com as the test:
So, we see "example.com" in the middle there. What we want is to generalize this URL for Flagfox using the custom actions syntax:
This, unfortunately, doesn't work. Took me a bit to figure out what this site wants; it's the "%3a" part that's the issue. Unescaping that back to a colon gets things working. Here's two working actions, one for looking up by domain name and the other for looking up by IP:
MX Toolbox (domain)
MX Toolbox (IP)
You can select and copy the two actions above and paste them straight into the Flagfox options dialog (ctrl+v or use the drop down menu next to the add actions button), or you can just drag them into the options dialog. (or of course, just paste them into a new action one at a time)
Took a little extra step this time because the site is doing something unusual.