HTML Header. If you need to insert some HTML in all the pages of an app that use this plugin, you can enter the HTML here. Analytics services, for instance, usually require to add a HTML snippet on all pages. If you need users to be able to enter a key in an HTML snippet (for instance a Google Analytics tracking code), you can add the dynamic key in your snippet between the sequence of characters _*_, such a _*_API KEY_*_. Once a user uses your plugin in their app, this key will be a setting to be filled in the Plugins tab, and the key will be replaced by the actual value in Run-mode.