In case any one wants the solution (it took me all day to figure it out), here is my discussion post HTML widget code, which depends upon an exposed prompts.json file generated from my YAML data:

<div id="prompt">Refresh the page if the discussion post prompt has not loaded here.</div>

<script>
getText("prompts.json");
const cid = {cid};

async function getText(file) {
  let response = await fetch(file);
  let data = await response.json();
  let messageObj = data.find(item => item.cid === cid);
  let message = messageObj ? messageObj.message : 'Discussion post prompt not found. It may not be ready yet, but will be a few days before the relevant class.';
  document.getElementById("prompt").innerHTML = message;
}
</script>

Thanks, @baris, for the pointer.

That's very cool! I haven't worked with NodeBB's widgets before, but it might be a little tedious to create HTML widgets for each prompt through the ACP. I'm looking for a scripted solution, as what i have in the back is a YAML file containing all of the prompts with the cids of the category to which they need to placed. Using a widget instead of a pinned post sounds great, as long as I can figure out how to script it.

How could I show the content of a topic I've pinned at the top of a category? I'm using the Harmony theme on my NodeBB forum, and I see that when I open a category pinned topics appear at the top, but I need to click on them to view the content. I'm a professor and I'm trying to set up a discussion for my students in such a way that when they open a category they first see my discussion prompt with some guiding comments and questions for the discussion.

Kudos to the NodeBB team for creating such amazing APIs, by the way. It has freed me from relying on my crummy LMS.