How can I read a POST request body which is JSON and get a property? I need to read a property and use it as a variable in proxy_pass.

Pseudo code:

$post_request_body = '{"account": "test.mydomain.com", "more-stuff": "here"}';
// I want to get
$account = "test.mydomain.com";
proxy_pass $account/rest/of/url/here;