This AppleScript, designed to be used with MailTags and Mail.app, assigns one or more keywords to the selected messages.

mailtags-assign-keyword

-- This script assigns MailTags keywords to the selected messages
-- Set some default values to be used later in the script
property assignedKeywords : {"Personal"}

-- Handler called when running script from script menu
on run
  tell application "Mail"
		set theSelectedMessages to selection
		if ((count of theSelectedMessages) < 1) then
			beep
			return
		end if
		using terms from application "MailTagsHelper"
			repeat with theMessage in theSelectedMessages
				set keywords of theMessage to assignedKeywords
			end repeat
		end using terms from
	end tell
end run