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?
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).
(An "email me a payment link" button would almost certainly get rejected by Apple)
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.
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.