Need help on capture JSON response from server

Hi all,

I am now writting script for Worklight server test and have some trouble.

Here is webservice's response

/*-secure-
{"challenges":{"wl_antiXSRFRealm":{"WL-Instance-Id":"kfi30hpd23m5r397ll0qulvsok"}}}*/

And I need extracly value of WL-INstance-ID .

Here are my code when I submit to webserver

web_add_header("x-wl-app-version","1.0");
web_add_header("x-wl-platform-version","7.1.0.0");

web_submit_data("init", 

        "Action=">XXX/.../init", 
        "Method=POST", 
        "RecContentType=application/json", 
        "Referer=">XXX/.../browser.html", 
        "Snapshot=t33.inf", 
        "Mode=HTML", 
        ITEMDATA, 
        "Name=isAjaxRequest", "Value=true", ENDITEM, 
        "Name=x", "Value=0.8243676177830881", ENDITEM, 
        LAST);

And Please refer to response from HTTPRequester. 

Thanks for your help