How to install Let's Encrypt certificate for domain alias in Plesk

Question

How to install SSL certificate for domain alias in Plesk?

Answer

A separate certificate only for the domain alias cannot be created or uploaded. Instead, create a new Let's Encrypt certificate for the main domain and specify the alias to secure:

I. Log in to Plesk and install SSLIt! from Extensions catalog if it is not yet installed.

II. Create a new certificate:

1. Go to Extensions > My Extensions tab > SSLIt! and click Install button in Recommended extensions next to Let's Encrypt icon.
2. Go to Websites & Domains > SSL/TLS Certificates and click Get Certificates button. Scroll down to "More options" and click Install button next to "Install a free basic certificate provided by Let's Encrypt"
3. Specify email address of domain owner. 
4. Specify if you want to include an alternative domain name for the domain and each selected alias, for example: www.example.com for example.com. It is recommended to select this checkbox.
5. Specify if you want to include webmail, for example: webmail.example.com. It is recommended to select this checkbox.
6. Select the domain alias to secure and click Install

III. Secure the main domain with it's aliases:

1. Go to Websites & Domains and click Hosting Settings.
2. Select the SSL/TLS support checkbox.
3. Select the newly issued Let's Encrypt certificate from the Certificate menu, and click OK.

Additional information

To secure domain alias with a paid certificate, contact a certificate provider to add the alias name to the certificate as the subject alternative name (SAN). Then, secure the main domain with this certificate as described here.