Just make an app that automatically connects to your email, gets and parses the email (there can be a specific format) and asks "would you like to sign in to slack?" And you can just press "ok" and you'll get in. To make it even better, you can make it so that the email is signed by a pgp key you've previously trusted ("would you like to sign into trusted provider 'slack'?"), to make it resilient against phishing emails, and for extra protection the slack sign in page can show a random series of 6 digits, which is also emailed to you, so a phishing email wouldn't have access to those 6 digits; and you can compare those to what the app shows you.