Redirect to www not working

Hi everyone,

I have a subdomain eg (test.test.com) created in plesk with all necessary DNS records, including (www.test.test.com)

My need is when accessed the subdomain (test.test.com) it should redirect & load like (www.test.test.com)

Tried with all redirection codes through (web.config) & (.htaccess) nothing clicked.

Can anyone suggest me with tips or solution to resolve it.

WEBSERVER: IIS running with plesk...

Redirect to www not working