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"]}
globals
{"url": globals.url}
{"url": globals["url"]}