In your domain name resolution panel (DNSPOD is taken as an example in the following figure), add a CNAME record.

Fill the copied A into the host record on the resolution panel, and B into the record value. Select CNAME as the type. Save it.

After setting, the following URL can be used to check whether the setting is successful: //www.whatsmydns.net/#CNAME/

After setting, click Submit on the order page. The CA server will verify your order and issue a certificate after it passes. Generally 5-10 minutes