Ajax 如何在Fetch中将对象作为数组发送，而不运行循环和映射来转换它？

我有以下jqueryajax->POST

        let url = "engine/api.php";
        let act = "reset_pin";

        
        let url = "api.php";
        var postvars = {};
        postvars.act = "move";
        
        $.post(url, postvars, function (data) {

现在我试着在fetch中这样做。。。但它不起作用。。我得到的是：

  var postvars = {};
        postvars.act= "move";
        fetch(url, {
            method: 'POST',
            headers: {
                "Content-type": "application/x-www-form-urlencoded; charset=UTF-8"
                },
            body: postvars
            })

服务器未正确获取PostVar