Make the trigger start a script which looks something like this. This just a sketch, not a working script. Syntax not guaranteed.
; this script shows the countdown
_timer=600
#loop
?condition: goto "end"
hint format ["Time remaining %1s", _timer]
~1
_timer=_timer - 1
? _timer <=0: goto "timeout"
goto "loop"
#timeout
whatever
exit
#end
whatever
exit
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Unrelated PS - Thanks Chris, it's nice to be back