That’s a spoiler tag. You trigger it by three colons followed with a space then the word “spoiler” and end with 3 colons on a new line at the bottom. If you want to modify what the dropdown is called, just add that word preceded with a space after the spoiler
For example, in the above post the structure was:
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As a trumpet player at one point, I do have to acknowledge that the French horns were widely considered the best cuddlers due to the practice they got holding their own instruments.