Need help with simple Rewrite

Hello, I don't really work with IIS and had a customer that was requesting a login portal to be active on their website.  We are currently hosting the site in our IIS 8.5 local server and was looking for help on how to configure Rewrite.  I'm sure this is simple but since I have never done it and don't now the correct dialog, I'm just spinning my wheels reading posts trying to figure it out.  Here is what I'm looking to do and if you can provide step by step how I can achieve this in IIS with Rewrite, I do have the Rewrite module already installed. I have created a DNS Foward Lookup Zone on my domain controller for portal.mydomain.com that points to my local IIS server. I have configured a DNS A record with my ISP pointing to the local IIS server and my firewall configured for HTTPS and everything is working. When I type https://portal.mydomain.com/SITE_ONLINE outside of the office I'm able to get to the login screen, I would like to make the site readhttps://portal.mydomain.com/Login

Thanks Ryan.