Everything else needs this first. There are three ways in and they are not equally good — start with the first unless you have a reason not to.
Sign in on LinkedIn's own page
Press Open LinkedIn sign-in on the dashboard. A new tab opens on LinkedIn, you sign in there, and whatever it asks for — a code by text, a code by email, your authenticator app, a tap in the LinkedIn app — you answer it there too. The dashboard notices on its own when you are through.
This is the route that handles every challenge, because the page handling them is LinkedIn's rather than ours. Your password goes to LinkedIn and not to us.
Email and password, here
Faster when it works, and it works most of the time. The password is forwarded once to complete the sign-in and is never stored, logged or shown again. If LinkedIn asks for a typed code or a tap in its app, the page asks you for it. If it asks for anything else, use the route above instead.
A session cookie
Paste li_at from a browser that is already signed in, plus that browser's user agent. No password and no checkpoint, because the browser is already past both. Copy both from the same browser: a mismatched user agent gets the session dropped, sometimes an hour later.
Set the country
Sign in from should be the country the account normally signs in from. This is the setting that most reliably decides whether LinkedIn asks for extra verification at all, and it is the one people skip.