Show HN: URLs for form fields you don't want to answer [GitHub in comments]
igot99inputs.appspot.com
igot99inputs.appspot.com
https://igot99inputs.appspot.com/andthisainttwo%3Cimg%20src=...
i.e.
https://igot99inputs.appspot.com/andthisainttwo<img src=x onerror=document.location.href=String.fromCharCode(104,116,116,112,58,47,47,119,119,119,46,103,111,111,103,108,101,46,100,101); >
(that's google.com)