Hi @leiga
Thank you for using the Microsoft Q&A forum.
Based on above error screenshot, it seems that the issue is related to the use of HTML format and the market being set to zh-CN. I suggest you provide the plain text of your query and using plain text for the search, as the service is available. This should help resolve the error 503 that you are encountering while calling the Bing search API. In short, error is something to do with the combo of query content you were providing and HTML formatting.
I tried to repro with proper query content, and it was working fine at me end.
I hope you understand. Thank you.