Trigger over the relevant area
Activation box: east present
Condition: (count thislist <= 1) and (alive capturedOfficer)
Type: End#1
Syntax not guaranteed. You will need other triggers to cover other possibilities e.g. if the officer you are supposed to capture is killed before he is captured, or if he is killed after he is captured.
Read the online version of the command reference for the synax. Check the Unofficial command ref guide as well. The best way to get to understand a command is often to createa little test mission and play with it.