That’s not really a good solution at all…
To install a Plugin you need an Account and a registered domain - If you want to test a plugin for your shop but you do not want to test it in the live shop you would be best adviced to follow the original post from wontfix.
You install a virtualbox with vagrant and put a copy of your live-shop into the local virtual maschine.
It will give you all required info about ports and IP to address it locally upon installation.
After that you can edit your host (local) file to route the URL of the shop not to the live-Shop but instead to the local maschine where your test-shop is waiting.
Now that you can use the URL that was formerly linked to the live shop for your local installment, you are also free to download plugins to the test shop as if it were the registered live shop itself.
Once you are sure that the plugin is safe to install you can switch the host file back to the live shop (add a # in front of the entry to nullify the effect but keep it for later) and install the plugin in the real backend this time (probably from the purchased plugins because „your domain already bought it“ while you were in the local shop).
Takes a lot longer to set up but is the clean way to do it and it will most likely come with a bunch more advantages since it’s basically just like the real shop.