The Tableau parameter format are described in the most detail here: https://github.com/dataiku/dss-plugin-tableau-hyper/#parameter-preset
So for URL and Site ID:
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.
Let us know if you have any questions!