Composer is the main package manager responsible for extending Magento 2.
Each of the examples below is required to add the repository and the key pair so that you easily add and upgrades extensions from the vendors. Please note each vendor will provide you with a method to obtain your public and private keys usually in your account area. These must replace the placeholders (PUBLIC_KEY PRIVATE_KEY) in the below commands.
If you are purchasing your extensions from Magento Marketplace you can use this guide to obtain your access keys. Alternatively most vendors will also provide you with composer access keys and instructions which will be very similar.
On your MDOQ instance, click 'Support' then 'Web SSH' then
cd ~/htdocs composer config --global http-basic.repo.magento.com PUBLIC_KEY PRIVATE_KEY
Other common extension vendor Repo's
cd ~/htdocs composer config repositories.aheadworks composer https://dist.aheadworks.com composer config http-basic.aheadworks PUBLIC_KEY PRIVATE_KEY
cd ~/htdocs composer config repositories.amasty composer https://composer.amasty.com/community/ composer config http-basic.aheadworks PUBLIC_KEY PRIVATE_KEY
cd ~/htdocs composer config repositories.mageworx_packages composer https://packages.mageworx.com/ composer config http-basic.mageworx_packages PUBLIC_KEY PRIVATE_KEY
cd ~/htdocs composer config repositories.ebizmarts composer https://packages.ebizmarts.com composer config http-basic.gitlab.ebizmarts.com token TOKEN
cd ~/htdocs composer config repositories.wyomind composer https://repo.wyomind.com composer config http-basic.wyomind PUBLIC_KEY PRIVATE_KEY
Enter your credentials when you install the module and choose Yes when asked if you wish to store your credentials
cd ~/htdocs composer config repositories.zero1 composer http://packages.zero1.co.uk/ composer config secure-http false
If you have successfully added the repo and the necessary keys, and you are on MDOQ managed Magento hosting, you should then be able to search for the extension via the MDOQ 'Install Module' option in the Magento Tool Belt > Actions, alternatively you can use the CLI tool.
REMEMBER, when you have finished, use the Git option to 'Git Push' to ensure you commit your composer changes (usually in auth.json, composer.json and composer.lock)