How to pass a variable into JSON Request in a file

Hi I am trying to pass the variable value in a test case into JSON request body which is in a separate file.
eg “${appId}” which is in my request json body. I created the variable in test case and want to pass a value to it.

use SimpleTemplateEngine:
http://docs.groovy-lang.org/docs/next/html/documentation/template-engines.html#_simpletemplateengine