You might want to try seeing if there is something there in the app to activate your subscription
or
look for an eMail in your inbox from the app developer
or
Is this the website online (help section) connected with the particular App you subscribed to:
https://help.audible.com/s/article/how-can-i-sign-in-to-the-audible-app-on-ios-device?language=en_US
or
How to contact an app developer – Apple Support
_________________________________________________
If needed:
If the App came from the App Store:
App Store Support webpage:
(See the "Get Support" link at the bottom of their webpage (below)
App Store - Official Apple Support
All the best :-)