Most(every?) app that I’ve signed up for requires an email address. Some apps then send a verification or welcome email. If you mention and link to paid subscription upgrades in that email, will Apple pull your app?
As Spotify points out, and I've seen first hand, Apple prohibits you from allowing users to create accounts or enter contact information if your service has a paid option and you don't use apple's IAP.
So you couldn't even get to the point where you can send an email without first falling in line with IAP.
And according to my understanding of the Spotify timeline, and I haven't see this because I haven't published to the platform in a while, Apple now prohibits you from targeting your iOS users with upgrade communications (even if they didn't sign up via iOS, which they didn't since that's prohibited).
“We’ve just sent a verification code to the email you provided, please type the 4 digit code here:_ _ _ _”
It could even be 2 digits (the same 2 digits for everyone), you’re just looking to get their eyes on the email letting them know additional paid subscriptions are available on your website.
Let's assume you purposefully crafted the email or app to break the code autofill built into iOS.
I think you'd end up confusing users. Most people will go to their email expecting to see a verification code in big bold type. If you make the code less obvious, they'll get frustrated ("I came here to get a code, why am I now doing something else?") It's awful UX at a particularly business-critical moment.
And this is all assuming Apple doesn't see through your ploy and just reject the app.
