fired = "if ((_this select 1) == ""KTE_Throw"") then {_this exec ""\path_to_your_addon\sciptname.sqs""}";
What this does is it checks to see if the weapon used is the KTE_Throw weapon (if I remember correctly that's what I named the throw weapon in my addon), and if so it runs the startup script for the grenade fusing.
You could easily check so it's an ammotype instead, by using _this select 4 instead. You could also check against multiple types of weapons or ammo, by doing something similar:
if ((_this select 4) in [""AmmoType1"",""AmmoType2"",""AmmoType3""]) then {}
Note that you need to include double-double quotes (two on each side) with the ammo and weapon classnames, since otherwise you'll get a syntax error. This is when adding them in the config.
You could just as easily add them in a mission with the addEventHandler command, the arguments and parameters are the same.