1. Search for the Wand of Illusion ABILITY (The one the Item has in the Ability data field, I think it's "Illusion (Item)")
2. Press CTRL + D (This enables seeing Raw Codes)
3. Write down the Illusion Ability RawCode
4. Create the trigger with any order
5. Backup the trigger (Create a Copy)
6. Select one of the triggers and go to Edit -> Convert to custom text
7. Search there for something like "UnitIssueOrderByID" or something like that. That's the Custom code of the action you want to do.
8. The action uses arguments: UnitIssueOrderById(TargetUnit, OrderString) or something like that.
9. Replace the order string of your action with the Raw code of the illusion this way: 'A0H2' with the '''''s
<< EDIT >>
Instant Cast Orders (like Stop)
By string
native IssueImmediateOrder takes unit whichUnit, string order returns boolean
By Id
native IssueImmediateOrderById takes unit whichUnit, integer order returns boolean
Point Orders (Like Move)
By string
native IssueInstantPointOrder takes unit whichUnit, string order, real x, real y, widget instantTargetWidget returns boolean
By Id
native IssueInstantPointOrderById takes unit whichUnit, integer order, real x, real y, widget instantTargetWidget returns boolean
Target Orders (Like Illusion)
By string
native IssueTargetOrder takes unit whichUnit, string order, widget targetWidget returns boolean
By Id
native IssueTargetOrderById takes unit whichUnit, integer order, widget targetWidget returns boolean
<< EDIT >>
This is the action to cast Item Illusions ('Alil')

Custom script: call IssueTargetOrderById(GetTriggerUnit(), 'Alil', GetSpellTargetUnit())
call IssueTargetOrderById(GetTriggerUnit(), 'Alil', GetSpellTargetUnit())
'Alil' is interpreted by the game as an integer.
You can use this whenever you want to know the Id of any order.
Jass:
call BJDebugMsg(OrderId2String(GetIssuedOrderId())) // This tells you the string
call BJDebugMsg(I2S(GetIssuedOrderId())) // This tells you the number