Pre-adds allows your fans to listen to one or more songs on Apple Music from an Album or EP before it is available for sale on iTunes. Bonus points, this also automatically adds the Album or EP to their library once it is available for sale.
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Pre-add is only available for EPs and Albums where at least one track has been opted into Instant Gratification. For more information on Apple's requirements for pre-add, check out this article: https://itunespartner.apple.com/music/articles/rights-and-pricing_pre-adds-for-apple-music
For more information on how to opt into an Instant Gratification track on iTunes (which you need to do in order to use pre-add) check out this article: