Some online services tend to use rtmp Servers like Wowza and similar. These servers often need you to authenticate yourself with your username and password instead of a stream key.
To enter a Username and Password you just have to add them to your "profile name.ini" which is located in the %appdata%\OBS\profiles folder. If you are not using a special profile, just use the untitled.ini, search for the section [Publish] and add the following two lines:
UserName=[type username] Password=[type password]
to this section. Of course without the [ ] brackets. Make sure OBS is closed while you change your profile.ini, or restart OBS afterwards. Then setup your server address in the Broadcast settings as usual and start streaming.