Syntax region not working as expected

Hello, new to the group.

I am trying to prevent the vim spell check from highlighting citekeys in Markdown texts (in the format [@Author:1999ab, p.\ 99]). for this, I set up a syntax line in .vimrc:

<code>
syntax region ExCitation start="\[@" skip="\\\]" end="\]" contains=@NoSpell
</code>

However, nothing comes out of it. Am I doing something wrong?

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