B4J Question Curl and poststring

I am using the J.PostString for posting URl's, and all is ok

but now I need to post also body:
POST /v1/conf/gatewayGroups/gatewayGroupName

And in the body
{

"gatewayGroupName":"xxx",

"gatewayPlatform" : "yyy",

"gatewayMode": "zzz",

"failMode":"ttt",

"isCluster":"false"

}

how do I do it? I tried few things and I am getting error from the server