Entering text into a secure text field

How do I enter text into a secure text field in Safari using System Events?

tell application "Safari" to activate
tell application "System Events"
	tell process "Safari"
		set value of text field 1 of group 3 of UI element 1 of scroll area 1 of group 2 of window "User" to "user name"
		-- set value of secure text field 2 of group 3 of UI element 1 of scroll area 1 of group 2 of window "Password" to "password"
	end tell
end tell

Thanks
Pirri