You can use online tools like CloudConvert to create RAR archives by uploading your files and choosing the RAR format.

Based on your request, it seems you are trying to create a (possibly named 21928.rar). The best way to do this is by using WinRAR on Windows or a similar archiving tool. How to Create a RAR File (Windows)

Are you trying to or are you working in a specific software ? Is "21928" a file name, a document ID, or a code?