Sorry about that! Looks like the API docs are a bit broken at the moment. The payload you’re looking for needs to have the card ID in place of additionalProp1, e.g.
Which should work. After that though, an issue you will bump into is that doing this will not then update the HTML in lockstep – just the markdown. On the Supernotes frontend itself we are producing both and sending both to the API, but the API will not do that for you at the moment.
That being said, I will see if we can’t add this quickly as part of the 2.2.5 update (and try to fix up those docs a bit!) so that you don’t need to generate the HTML yourself. In the slightly longer term (over the next couple months), we plan to completely overhaul the developer docs to be much more comprehensive, helpful and accurate than they are now.
But thank you for helping us test the API in the mean time