Use the following endpoint:
https://SITE/discourse-ai/summarization/t/TOPIC_ID.json
Eg:
curl https://meta.discourse.org/discourse-ai/summarization/t/325562.json
Be sure not to include the stream parameter. The stream parameter will cause the message bus to deliver new summaries, which is a more complex protocol.
Also, be sure to use an API key for a user in a group that is able to generate summaries.