Currently, Discourse does not have any built-in features to auto-populate the email field on the login page using a URL parameter (like ?email=person@gmail.com ).
If you’re looking for a user login flow where users don’t repeatedly have to input their credentials, options like Single Sign-On (SSO) or DiscourseConnect can be configured for fully automated login processes.
For reference, you can see our official Discourse SSO and DiscourseConnect documentation here: Setup DiscourseConnect - Official Single-Sign-On for Discourse (sso).
There is also a Discourse OAuth2 Basic that allows you to use a basic OAuth2 provider as authentication for Discourse.