If your company or organization uses single sign-on (SSO) for Bitly, an admin for your account created a special login page for you to use. The URL for this page looks like this: [subdomain].bitly.com. If you don't know yours, contact an admin for your Bitly account.
To log in via SSO, go to your organization's login page and sign in using your organization credentials. You'll then be taken right to your Bitly account!