Installation can be done via the Universal Plugin Manager. Search for 'Notifications for JIRA' in 'Find new add-ons', and select 'Free trial' or 'Buy now' to install the plugin.
If you don't have JIRA Service Desk installed, or you have the Servicedesk Automation plugin disabled, you may get a warning that some modules couldn't be enabled. This is normal since these modules depend on the Automation features of JIRA Servicedesk. The plugin will continue to function as a workflow post function.
Don't forget to enter the general configuration after installing, otherwise the plugin won't work.