Make sure the trigger activation is only once (not repeatedly), then in the on activation field put:
thislist exec "script.sqs"
Then in your script make the first lines this:
_unitarray = _this
_unit = _unitarray select 0
.....rest of script is done by _unit
The reason for the _unitarray is because thislist is always an array even if it is only one unit. The _unit = _unitarray select 0 converts that array into a singe unit, or the first unit if somehow more than one unit enters the trigger at the exact same time.
Hope this helps you.