Yes, It is possible to enable SSL Certificate on the main domain in Shared hosting.
Here, we can enable both Free and Paid SSL certificates.
For Free SSL Certificate (Auto SSL), we can enable it from WHM Panel. We will have to Login into WHM Panel -> Manage AutoSSL -> Manager Users -> Enter the username -> Tick the option Enable AutoSSL. Once you select it, the SSL process will be initiated and installed on the domain name. You can check the log of the SSL Certificate from the logs option.
For Paid SSL Certificate, we need to generate a CSR key from the server with the provided details. After then, we need to approve the SSL Certificate request through an email account. Once the SSL Certificate request is approved, we will receive an SSL Certificate in the THS. We will inform the client to share his convenient time because we need to assign a dedicated IP address to the main domain to install an SSL Certificate. Also, the client needs to update the mail records from the DNZ zone. DNS changes may take 12 - 24 hours to propagate around the world. Once the client provides a convenient time, we will assign a dedicated IP address and install an SSL Certificate on the domain name.
We will share the IP address to the client and ask him to update the mail record from the DNS Zone.