Moved from Fritzbox to speedport

I took a look at your manual. According to page 66 there should be preconfigured services, including http://DynDNS.org. Select this service, and you should not need to enter an update URL.
If not, your update URL is most likely:

members.dyndns.org/nic/update?myip=<ipaddr>

The rest is as Cry Havok said.

domainname: h4t3t4nk.endofinternet.org
Update URL: members.dyndns.org
Username: Your DynDNS username
Password: Your DynDNS password

thank you, RotBlitz! only your answer helped me with the Update URL. my older Fritzbox did not work using the firware http://dyndns.org client. I guess it has something to do with the name change from Org to COM