Well, here’s the go:
1: trigger_multiple (this is a trigger brush which you give a ‘trigger_multiple’ key )
- target: portal_scripter (this could be the name for your target_script_trigger)
2: target_script_trigger (this should be a little yellow box)
- scriptname: axis_portal (‘chapter’ of the script to read)
- target: activate (part of the ‘chapter’ to read)
*targetname: portal_scripter (so now the trigger multiple is linked to the script trigger
Then your script looks like this:
axis_portal / this is the scriptname as seen in the target_script_trigger
{
spawn
{
}
trigger activate /this is the part it needs to read or execute... it doesn't matter how you call it, as long as your target from target_script_trigger matches this name
{
trigger game_manager axis_win
}
}
and the rest of the script is what Masterkiller suggested (well the script above is the same, but with some explanation)
What you did here:
- One trigger_multiple. It has a targetname called ‘axis_portal_box’
- One target_script_trigger. It has a scriptname (axis_portal) and a target ‘axis_portal_box’
is link the target_script_trigger to the trigger_multiple, that won’t do the trick 
(targetname is the name of the entity which is selected/activated in radiant, target is for linking it to another entity or to ‘aim’ it (can only be used with the target_script_trigger entity) to the correct part of the script to read)
I hope I’m a bit clear and it works out for you 
EDIT: Additional explanation
of course you can play with the trigger_multiple as well
depending of the storyline of your map, you can hide the trigger before players completed a certain objective (to avoid cheat ending the map)
or only allies or axis can activate the trigger. Because as it is now, both teams can end the map when triggering the trigger_multiple
If you need further information about scripting, you can always ask here or pm or visit cool websites or whatever 