To use Delta Chat with a T-Online email address, you need an app password. A (german) guide how to set this up is here: https://www.telekom.de/hilfe/festnetz-internet-tv/e-mail/e-mail-adresse-passwoerter-und-sicherheit/passwort-fuer-e-mail-programme-einrichten
You need to enter an additional password for external access in the web interface settings. To get to the settings, go to the T-Online webmail interface, click on the wheel in the top right, and then click “Passwörter” on the left. You can now set a password at “Passwort für E-Mail-Programme ändern”.
Enter your password twice to set it, and confirm it. Now you can login with this password in Delta Chat.