Java Selenium - how to test browser network -> payload?

Question

Java Selenium - how to test browser network -> payload ?

I have following code that works:

WebDriver             driver;
BrowserMobProxyServer proxy;
Proxy                 seleniumProxy;
        
        
    
proxy = new BrowserMobProxyServer();
proxy.start(8080);
seleniumProxy = ClientUtil.createSeleniumProxy(proxy);
String hostIp = Inet4Address.getLocalHost().getHostAddress();
seleniumProxy.setHttpProxy(hostIp + ":" + proxy.getPort());
seleniumProxy.setSslProxy(hostIp + ":" + proxy.getPort());
proxy.setHarCaptureTypes(CaptureType.RESPONSE_CONTENT, CaptureType.REQUEST_CONTENT,
        CaptureType.RESPONSE_HEADERS, CaptureType.REQUEST_HEADERS
        );

How to test network -> payload in chrome browser ? I have to test several fields in payload (key, value pairs). Payload resembles json file.

Lets say I have to verify the value of tmax equals 3000. How to do this ?