Email questions as follow to Techsupport@xiaohongshu.com.
Apply for sandbox -- Send email with subject as [Store name in RED] + [apply for sandbox], please cc your Account Manager in RED.
Technical questions -- Send email with subject as [Store name in RED] + [API Questions].
Error in API -- Send email with subject as [Store name in RED] + [API Error], in content please provide full HTTP request, with URL, Header, body(data) and Response data.