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Why wouldn't you want to include a link to your help desk? If it's a resource that visitors will use then you should include it. As for a nofollow link, this is just going to stop link juice being passed to your hepdesk page, but other links on your page will pass less link juice as a consequence, so without more information, it doesn't seem to make sense to do this. If there's a reason why you don't want visitors to access the help desk then don't include the link at all.