Setting up redirects is an advanced task and should be considered carefully. This is usually a job for someone very savvy or even a developer.
preFIX sites will have limited FTP access and where the web.config would normally be accessible, it will not be in preFIX. Instead you will find a folder within your site's FTP called 'config' and redirects will be added in the RewriteRules.config file.
Using FTP, locate and edit; config/RewriteRules.config