Set the activation hook for a plugin.
When a plugin is activated, the action 'activate_PLUGINNAME' hook is called. In the name of this hook, PLUGINNAME is replaced with the name of the plugin, including the optional subdirectory. For example, when the plugin is located in wp-content/plugins/sampleplugin/sample.php, then the name of this hook will become 'activate_sampleplugin/sample.php'.
When the plugin consists of only one file and is (as by default) located at wp-content/plugins/sample.php the name of this hook will be 'activate_sample.php'.