I usually just click the UI button which gives (a bit confusing) template like this:

spoiler
___

then I replace second spoiler with the always-visible text and the ___ with the hidden content, ending up with Markdown like this:

star wars secret
this is an example so i won't actually include the spoiler

(ie. the ::: spoiler part is fixed, then the title which will appear prior to revelation, then the "secret", then :::)

...which renders like this:

just a normal text