The AutoComplete feature is turned on by default when:
A new user creates a profile.
You are an existing user and you click the Defaults button on the Preferences dialog.
To manually invoke the AutoComplete list box for an identifier
Position the cursor at the appropriate position.
Press CTRL+space.
You can configure this sequence by using the Editor Key Bindings sheet of the Preferences dialog.
The text to the left of the cursor is used to select the entries in the list box appropriate for the current content.