The issue luceos is: how the user can install the package manager without access to SSH?
@redhq do you have access to SSH?
If so you can install the package manager as suggested by luceos.
composer require flarum/package-manager:"@beta"
If you do is suggested to change (edit) also the file composer.json in the root flarum, you need search the line "prefer-stable": true, and change to "minimum-stability": "beta",
Then after followed this steps you can login in the Flarum backend, enable the extension Package Manager.
With this extension installed you just need to find the extension you want install and paste into the dedicated field the command to install the extension you desire.
