SERVER_URL is the URL of your Tableau server without subpaths.
For local Tableau servers, an example would be: https://www.MY_SERVER.com. For Tableau Online, an example would be: https://10ax.online.tableau.com/.
SITENAME is the subpath of your full site URL (also called contentURL in the REST API). MYSITE would be the site name of https://10ax.online.tableau.com/MYSITE.