gethttpsforfree.com / lets encrypt / account private key location

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My domains are for example: xyz-abc1.com and xyz-abc2.com. For the website xyz-abc2.com I want to generate a ssl-certificate over gethttpsforfree.com. For "Step 3: Sign API Requests" I need the location of my account private key. With plesk I've generated a csr request and a private key for this domain. In which folder can I found my private key for the domain xyz-abc2.com?