The same works for me. Are you sure you don't have some js errors? Try to run the same as html and check it in the browser console for errors.
Also, try to send just this through your postman, does it pass for you?
{ "template": { "content": "<script>window.JSREPORT_READY_TO_START = true;</script>", "engine": "handlebars", "recipe": "chrome-pdf", "chrome": { "waitForJS": true } } }