You can do it like this

<details>
  <summary>Short Summary</summary>
  <p>text to hide</p>
</details>

But it will only work in reading mode, otherwise every time you click it, it will let you edit the text.

Fair enough
There is an identical post on r/Selfhosted so guessing a copy paste job

Looks like it supports mastodon at least
https://docs.postiz.com/providers/mastodon