json block as a property value

Define a test case level custom property say JSON_REQUEST_FILE and provide the file path. This is just not to have any hard-code stuff in the script. If it is property, then user can just change path there and avoid modifying the script.

Use forward slash ('/') as path separator even on windows.

Add a Groovy Script Test Step before your existing REST Request test step.

Below script would set the file content to next test step. Throw error oterwise.

def fileName = context.expand('${#TestCase#JSON_REQUEST_FILE}')
def fileContent = fileName ? new File(fileName)?.text : ''
if (fileContent ) {	context.testCase.testStepList[context.currentStepIndex+1].httpRequest.requestContent = fileContent
} else {
   throw new Error('Could not find property JSON_REQUEST_FILE or file, check it')
}