Apparently all you have to do is log into battle dot net and claim it…
So far what I can see the deal is available until November 18…
Battle dot net announcement here:
https://news.blizzard.com/en-us/blizzard/22649418/destiny-2-pc-free-on-battle-net-through-november-18
Gift page here:
https://us.account.blizzard.com/gifts/?blzcmp=blizzard-news
Going to test it out soon.
EDIT:
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Created Battle dot net account, confirmed with email subscription confirmation.
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You need to use a valid phone number for SMS confirmation on your account if you are creating a new account. (I had to do this.)
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go to the gift page link above, and click claim. (You do not need the battle dot net client installed to claim your free copy.