Here's an updated version of the AppleScript that will run on Big Sur:
tell application "System Events" to tell the front menu bar of process "SystemUIServer"
tell (first menu bar item whose description is "Siri")
perform action "AXPress"
end tell
end tell
delay 1
tell application "System Events" to tell the process "NotificationCenter"
set notifyWindows to every window
repeat with notifyWindow in notifyWindows
if description of notifyWindow is "system dialog" then
set elements to entire contents of notifyWindow
tell (first text field of first group of first UI element of first scroll area of notifyWindow)
set focused to true
set value to ASTypeToSiriQuery
perform action "AXConfirm"
end tell
exit repeat
end if
end repeat
end tell
Edit: I should mention, I'm not very good with AppleScript, so this is probably going to break in the future, but it works for now.