SSRS Configuration

I'm trying to configure SSRS to use email subscription feature. This SSRS server is hosted on AWS and I have an SMTP relay to use. This SMTP relay requires an userid and password to authenticate. I tried SSRS configuration manager and also the config file, but couldnt find how and where to use this user name and password for the relay. I would appreciate if someone can help me with the configurations. The same SMTP relay and account details are working fine when I'm trying to configure the Database email on SQL Server.