Just install this extension the usual way like any other TYPO3 extension.
Via Composer using Command Line
composer req nitsan/ns-helpdesk
Via Extensions Module
In the TYPO3 backend you can use the extension manager (EM).
Step 1. Switch to the module “Extension Manager”.
Step 2. Get the extension
Step 3. Get it from the Extension Manager: Press the “Retrieve/Update” button and search for the extension key ns_helpdesk and import the extension from the repository.
Step 4. Get it from typo3.org: You can always get the current version from https://extensions.typo3.org/extension/ns_helpdesk/ by downloading either the t3x or zip version. Upload the file afterwards in the Extension Manager.