You will need to drop a link on your Site Page which will have a label to help employees Log in via that .SFDC labels as "If you are Employee Log in here" .
The URL for this login will look like below
Now if you notice the site param has an Id there attached .You will need to get your site Id from going to Sites in Set up menu and inspect element on chrome browser or other browser to get the Site Id .
You can also query for this Id .I will recommend to use labels so you can configure each time .
Also you will need to encode the StartURL parameter (URLENCODE).