Livestreaming platforms not listed in the Outputs or Plugins can still be used as long as they support RTMP through the Custom RTMP output option.
To set up a Custom RTMP output, go to Outputs > Set up a new output, then click Custom RTMP.
On the Custom RTMP Properties window, enter your stream details - most importantly the RTMP URL and Stream Name.
You can often find the RTMP URL and Stream Name under the profile or channel settings of your preferred livestreaming platform.
These details can sometimes be found under different names. For example, RTMP URL can be called Ingest Server or Stream URL while the Stream Name is usually called Stream Key.
Once you are done filling out the details, click OK to save your Custom RTMP profile.
You'll be able to start livestreaming with it by going to Outputs then clicking the new Custom RTMP profile you have created.