OK, this is a separate matter from
we want to bypass the step where users have to click “Login” on Discourse
Discourse does have functionality to generate login links
so one could hook into that functionality via a plugin, but for security reasons using that link requires positive action from the user to verify they wanted to login, so that method does not meet your initial criterion.
My advice would be to make the page they land on match your Discourse instance theming so they don’t notice the redirect.