Still no support for USA numbers :(
Response from developer
Thank you for your feedback. As of our latest update, buying single tickets and 1-3 day tickets does not require authentication on the app so you can buy tickets without needing to receive a confirmation SMS. To access this mode, make sure to select "Continue as Guest" when prompted with the option to create an account. If you continue to encounter issues during the process, please contact us through our customer service so we can take a closer look at your case, at https://www.hsl.fi/en/customer-service/feedback. Sincerely, Pate / HSL