This post applies to Microsoft Web Application Proxy 2012 R2. I recommend that you upgrade to Windows Server 2016 that provides a built-in function within the Web Application Proxy 2016 to perform HTTP to HTTPS direct. See more here Updated post: This post has been updated 07/32/2017. Original post: 25.
I found this article useful for customizing the logon page for the Web Application Proxy.. http://technet.microsoft.com/en-us/library/dn280950.aspx