Is there a way to use read browser login info into an app?

I'm trying to figure out a way to make the login process easier for the people using our app, as they seem to forget what e-mail they used when creating their account and come screaming that our app isn't working.

I, myself, am not a developer, but I do help them and provide them with whatever info they need regarding our system, as well as doing the UX/UI part of the app. Nonetheless, I do have a basic understanding of programming languages and every info you could provide would help me a lot in sending me in the right direction.

To explain what this is about, to be able to use a certain subscription you payed for, you have to install an app and login there using the info you provided when paying (the top schematics in the photo attached below).

Client flow schematics: client flow schematics

Now, because the sign up process happens in a different medium (the browser) than the app, the people seem to instantly forget the info they provided until they get to the login screen in the app.

Considering the fact that a fair chunk of our user base are not at all experienced with this kind of services and are scared of having their data stolen or whatever, I've found that relying on the AppleID and Google Smart Lock did not help at all and we would need something simpler.

Is there a way we could save the login info from the browser (as long as this happens on the same device used for the app) and "link it" to the app so that we provide the new user with a much more simple login process (like the second schematics in the picture attached)?

Thank you and please excuse me if I posted this the wrong way (if there's a right way to do this).