Try all other injection operators to see if any of them is not blacklisted. Which of (new-line, &, |) is not blacklisted by the web application

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how should answer be written

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This threw me off too.
First to get the result you want in burp, only append the non url-encoded or url-encoded injectable character, without spaces or anything, in repeater.
Once you find out which character it is, try every possible operator and character names of it within the cheat sheet to submit as the answer.