[Kameramann]

так?

var myJson = {
 "client": "test",
 "key": "test",
 "func": "ping"
};
    var str = "string";
    console.log(JSON.stringify(myJson));
    $.ajax({
        url:"https://api.b2cpl.ru/services/api_client.ashx",
        type:"POST",
        data:{myStr:myJson},
      success: function(data) {
  console.log(data);
        success: x => console.log(x)

}
    });