Reader Magnets for Current Subscribers?

In just over 2 months, I’ll be publishing a book that includes a link to bonus material at the end, which will be offered as a reader magnet to attract new subscribers (as in, you get the bonus content when you join my email list). Any suggestions on how to offer this to current subscribers? I assume if a current subscriber puts their email into a subscription box again, it won’t work.

I could send it in one of my regular emails, but it’s the kind of content that will spoil the main book if they read the freebie first, so I’m hesitant to do that.

I could add a note on the sign-up page directing current subscribers to contact me for their free copy, but my reply won’t be instant, so I could see that being annoying for readers.

Is there a better way to do this that I’m not aware of? How do other authors pull this off?