Where in my OpenAI dashboard can I confirm this is the correct endpoint for me to be using (for my Wordpress chatbot)?
It is not the correct endpoint.
Assistants requires a header to make the API work, not a special URL.
You can read the CURL examples in the API quickstart - “Docs” in the platform site.
https://platform.openai.com/docs/assistants/quickstart?lang=curl
However, understand that Assistants is multi-call to place messages, to initiate a run, to poll to await a finished status, etc. You will need to track which thread containing messages belongs to which user or session, and more.
The Responses endpoint may be easier to use.