Once you have created an account, you can add a phone number to sign in via SMS. Follow these steps to activate and use SMS sign-in:

1. Sign in to your account.
2. Click the Account button at the top of the page, then select Account settings from the menu.
3. Choose the Phone option.
4. Enter the phone number you want to use for SMS sign-in.
5. Once you have added your phone number, you will receive a 6-digit sign-in code via SMS.
6. Confirm your phone number by entering the code in the text box that appears on the screen.

By completing these steps, you activate SMS sign-in. The next time you sign in, you can choose to sign in with your phone number or email.