Add the composer repository
composer config repositories.julienanquetil vcs https://github.com/julienanquetil/M2SendinBlue
composer require julienanquetil/magento2-module-m2sendinblue
composer update julienanquetil/magento2-module-m2sendinblue
This command enables the module that make up the M2SendinBlue extension
php bin/magento module:enable JulienAnquetil_M2SendinBlue
Once a module is enabled, the rest of Magento can "see" it. The last command tells Magento to actually install the module.
php bin/magento setup:upgrade