how to send file to sharepoint from another application?

I know how to upload files to sharepoint programmatically from CSOM but I need to do it from an application that I can't modify, this application only gives me the option of establishing a url where it sends the file, what url can I use? what code can I use in that url to deal with the file sent?

the application sends a xml file any time a survey is filled to an url, I want to include that xml file in a library and process the xml.