Add TrustedSite as a dependency by adding the following line to composer.json located under your Magento root folder (e.g. /var/www/magento):
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Run 'composer update'

Enable the TrustedSite extension by adding the following line to app/etc/config.php located under your Magento root folder:
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Run 'bin/magento setup:upgrade'

Once the TrustedSite extension is enabled, go to your Magento admin backend to create and/or log in to your TrustedSite account.

After setting up your account, the TrustedSite trustmark should appear on your store layouts (please allow up to several minutes for activation).