Filter sharepoint list with rest API on lookup field

function GetListData(select)
{
var siteURL = _spPageContextInfo.webAbsoluteUrl;
var listname = "MyListName";
var currentUserId=_spPageContextInfo.userId; //is a user number Then needs to use /ID
var url = siteURL + "/_api/web/lists/getbytitle('" + listname + "')/items?$select=ID,Name,Title,MyUserField/ID&$filter=substringof('"+currentUserId+"',MyUserField/ID)&$expand=MyUserField/ID";


$.ajax({
url: url,
method: "GET",
headers: { "Accept": "application/json; odata=verbose" },
success: function (data) {

var items = data.d.results;
alert(items.length);

for(var i = 0; i < items.length;i++) {
var item=items[i];

//console.log(item.Title);
}
},
error: function (data) {
alert("error");
}
});
}