To turn on the Script Assistant:

Choose Script Editor > Preferences.

Click Editing.

Click Use Script Assistant in the Script Assistant section of the pane.
You can have Script Editor automatically complete portions of the script you are typing by turning on the Script Assistant. Script Assistant observes the characters you type in the Script Editor window and provides suggestions to complete the code.
Choose Script Editor > Preferences.
Click Editing.
Click Use Script Assistant in the Script Assistant section of the pane.
As you type into the Script Editor, the Script Assistant looks for possible matches of code to recommend. If Script Assistant believes it knows what you are typing, it completes the code in gray. If you want to use the code as suggested, press the Esc key.
When several possible matches are found, an ellipsis appears next to the text cursor. Press F5 to see the list of suggestions to add to your script. Press the Up and Down arrow keys to select the one you want, and then press the Return key.