Matomo (Piwik) lets you measure several websites within one Matomo server. Each website added into Matomo has its very own ID Site (or Website ID). To find out the idSite value for a given website, log into your Matomo, then click on “Administration” icon in the top right of the screen, and go to the Websites page. In this page you will find the list of all websites. The website ID is on the left of this table listing all websites (below the website name).