Require the extension with composer:
composer require waldhacker/typo3-oauth2-client
Activate it either via command line or in the extension manager
./bin/typo3 extension:activate oauth2_client
Download the extension from TER and activate it in the extension manager.
Please note that the TER version comes with only the
as additional providers cannot be required by composer, if you want to use
a more specific provider to ease configuration, please load it yourself.