Registration
To get your login credentials, you have to send a SMS with the word open to 9526 (at least in Germany). After that you’ll get a SMS in return containing your login data.
Bookmark
You can add a bookmark to Safari with the following link:
https://hotspot.t-mobile.net/wlan/index.do?username=USERNAME@t-mobile.de&password=PASSWORD&strHinweis=Zahlungsbedingungen&strAGB=AGB(Replace USERNAME and PASSWORD with your data.)