As Postman is preparing your request it url-encodes the query parameters but leaves brackets out if you are not using globbing. However, you do not need to turn globbing off in JMeter to have JSON in the query parameters. You can encode the brackets too like so:

• '[' -> '%5B'
• ']' -> '%5D'
• '{' -> '%7B'
• '}' -> '%7D'

You can do it by hand or find a url encoder online. If you choose the latter, do not give the encoder Postman's partially encoded request as it has already been encoded once and will try to encode the existing '%' characters.

Note: I solved the problem as I was writing it but wanted to keep the question up because I want to learn more alternatives as I am pretty new to all this. *

Note 2: 'encode' word count: 8