Your expression is correct, try putting the expression into a text field field instead, you are using a drop-down.
Replace the second field with a text field, but use the same expression and 'Triggered By' settings.
You can then build an extended task and use that value in the details.
I tried the same express using a Text Field
I get the error
AutoFill expression validation failed. Field - 'Text Field 1'. Please check expression syntax
Expression is the following
Triggered by Printer (first drop down)
Works in the preview, but will not let me save or move on.