Magento 2: How to install a 3rd party private github repository extension bought from Magento 2 marketplace through composer.json

I have recently purchased an extension from the Magento 2 Marketplace and have downloaded the zip file from the downloads page as well. Now I want to install this extension through command line using composer.json because of the extension’s dependency on other 3rd party libraries.

If I just create a folder inside app/code and copy this extension there, the extension doesn’t work. I have also tried to copy the extension in app/code and then run “composer install” in the extension directory which creates a “vendor” directory with required files inside the extension directory. But somehow the downloaded extension is still not able to find the required libraries and classes.

The only option I can see is to do composer require and add the package in the core Magento 2 composer.json file but since the 3rd party extension is in private Github repository I cannot download the extension directly.

Any help in this regard would be appreciated.