トピック作成のための外部公開APIを作成する最良の方法は？

[maiki]

@Tim_Lee

My goal right now is to create an HTTP POST endpoint on the discourse server that creates a topic based on the request body.

I think we have what you are looking for, already: https://docs.discourse.org/#tag/Topics

@Tim_Lee

Is building a Discourse plugin the best way to achieve this?

Check out Discourse REST API Documentation, maybe what you want already exists.