How Can I Create a Multi-Line Input Dialog using Applescript?
One workaround is to make the default answer include a linefeed, but it will also be included in the default answer:

display dialog "" default answer linefeed




CocoaDialog supports multi-line dialogs:

do shell script "/Applications/CocoaDialog.app/Contents/MacOS/CocoaDialog \\
textbox --editable --title Title --button1 OK --button2 Cancel" without altering line endings
tell result
    if item 1 is not "1" then return
    set answer to text 3 thru -2
end tell




I couldn't get the text view to have focus when full keyboard access is enabled, even with ‑‑focus‑textbox or --selected.