Download and run the setup file. Then specify the URL where a HelpDesk instance will be created and credentials for Full Control access:
Follow the wizard steps.
If you specify a nonexisting site, it will be created automatically. For example, if you specify https://yourdomain.sharepoint.com/HelpDesk and HelpDesk doesn’t exist, it will be created for you.
We are not able to install HelpDesk unless scripting capabilities are enabled. Please make sure that you enabled scripting capabilities. Read this article for more information.
Once HelpDesk has been installed, you can go on to configuration.