i have a pretty difficult question here:
Is it possible to PRE - insert the email address in the corresponding email input from a GET request into the default wordpress form?
Here is a detailed reference how to customize the WordPress login form https://codex.wordpress.org/Customizing_the_Login_Form
Basically you can customize almost every aspect of the login form in terms of HTML.
If you want a quick solution without modifying code, you can register an additional Javascript that can read the URL query parameters from window.location.href and append it as email/username field placeholder/value attribute.
Here is a topic how to read URL GET parameters with Javascript How can I get query string values in JavaScript?