It is called a “Details disclosure element”.
You can type it directly in HTML:
<details>
<summary>Details</summary>
Something small enough to escape casual notice.
</details>
Appearance:
Details
Something small enough to escape casual notice.You can also find it under the “gear” tool tab as “Hide Details”.
The syntax is slightly different:
[details="Summary"]
This text will be hidden
[/details]
Summary
This text will be hidden