Postman - How to pass a global variable into JSON body

Question

If using a raw JSON body:

{
    "test key":"{{global variable}}"
}

On sending the request, the curly braces will be replaced with the value of the variable.

I think what you want to do is described here.

To use a variable you need to enclose the variable name with double curly braces – {{my_variable_name}}.

Double curly bracket is working in header parameter, url or inside JSON body. Inside tests you should use globals example: {"url": globals.url} or {"url": globals["url"]}

Yep, double curly brackets is the way to go to achieve this - make sure you have the latest version of Postman (though if you are still running 2014 code when this feature was introduced, shame on you!)

eg:

{
    "variable": "{{value}}"
}

See the second paragraph here in the Variables section of the documentation - it specifically mentions the request body.