Sharepoint - SharePoint 2013 REST - Create list item with user field and lookup field

It depends whether User/Lookup field value is multiple or not, the following formats are used when specifying field value via SharePoint REST:

• Single User/Lookup value format: FieldName: LookupIdVal
• Mutiple User/Lookup value format: FieldName: {"results": [LookupIdVal1,LookupIdVal2] }

How to set multiple lookup/user field value via SharePoint REST API

var listName = 'Tasks';

var itemProperties = {
    '__metadata': { "type": "SP.Data.TasksListItem" },
    "Title": 'New task',
    'AssignedToId': {"results": [12] }  //multi-valued User field value 
};

$.ajax({
    url: _spPageContextInfo.webAbsoluteUrl + "/_api/web/lists/getbytitle('" + listName + "')/items",
    type: "POST",
    contentType: "application/json;odata=verbose",
    data: JSON.stringify(itemProperties),
    headers: {
        "Accept": "application/json;odata=verbose",
        "X-RequestDigest": $("#__REQUESTDIGEST").val()
    },
    success: function (data) {
        console.log(JSON.stringify(data.d));
    },
    error: function (data) {
        console.log(data.responseText);
    }
});

How to set single lookup/user field value via SharePoint REST API

var listName = 'Tasks';

var taskProperties = {
    '__metadata': { "type": "SP.Data.TasksListItem" },
    "Title": 'New task',
    'AssignedToId': 12  //single-valued User field value 
};

$.ajax({
    url: _spPageContextInfo.webAbsoluteUrl + "/_api/web/lists/getbytitle('" + listName + "')/items",
    type: "POST",
    contentType: "application/json;odata=verbose",
    data: JSON.stringify(taskProperties),
    headers: {
        "Accept": "application/json;odata=verbose",
        "X-RequestDigest": $("#__REQUESTDIGEST").val()
    },
    success: function (data) {
        console.log(JSON.stringify(data.d));
    },
    error: function (data) {
        console.log(data.responseText);
    }
});