ASP.Net: Set custom error page for 400 bad request in IIS

You cannot customize the following HTTP error messages: 400, 403.9, 411, 414, 500, 500.11, 500.14, 500.15, 501, 503, and 505.

Refer Configuring HTTP Error Responses for more details.

So it's not possible with IIS.

You can refer the below link as an alternative.

https://serverfault.com/questions/257680/properly-handle-iis-request-with-percent-sign-in-url