Use of promo codes for multi platform app

I'm developing an app for iOS and Android. I want to give codes to potential customers to be able to test the app for free for a limited time.
Furthermore, I don't know if the customer has an iOS or Android device. Is there a way to work with promo codes inside the app? Using the App Store Connect promo code system would limit the codes to the Apple ecosystem.

I want to comply with the Apple review guidelines but something like this is an important marketing tool for my app.

Did you find a solution? I find myself discussing the same issue over and over again with the Review team. In our case, we don't even unlock any feature, we just use multiplatform codes to show different In-App-Products to the user, yet every iOS user still needs to make an App Store purchase. And still, the review team randomly rejects our updates without us even touching the coupon system they approved multiple times in the past...

Use of promo codes for multi platform app
 
 
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