SECTIGO Knowledge Base offers a detailed tutorial including screenshots https://support.sectigo.com/Com_KnowledgeDetailPage?Id=kA01N000000zFIw
For pro users, the short version below is focusing on the details of the certificate chain:
After validating the SECTIGO certificate request, SECTIGO is delivering your PositiveSSL certificate by email. The email includes an attached ZIP file that contains the following files:
EITHER:
<yourdomain>.ca_bundle
<yourdomain>.crt
OR:
SectigoRSADomainValidationSecureServerCA.crt
USERTrustRSAAddTrustCA.crt
AddTrustExternalCARoot.crt
<numbers>.crt
Plesk 12 is only offering two textfields for pasting your certificate. In order to paste your PositiveSSL certificate including the full certificate chain, copy the contents of your "your_domain_com.crt" / "<numbers>.crt" into the field Certificate. The CA-Certificate consists of SectigoRSADomainValidationSecureServerCA.crt appended by USERTrustRSAAddTrustCA.crt. The following content has to be pasted into the "CA-Certificate" field:
PositiveSSL CA-Certificate (Intermediate):
-----BEGIN CERTIFICATE----- MIIGEzCCA/ugAwIBAgIQfVtRJrR2uhHbdBYLvFMNpzANBgkqhkiG9w0BAQwFADCB iDELMAkGA1UEBhMCVVMxEzARBgNVBAgTCk5ldyBKZXJzZXkxFDASBgNVBAcTC0pl cnNleSBDaXR5MR4wHAYDVQQKExVUaGUgVVNFUlRSVVNUIE5ldHdvcmsxLjAsBgNV BAMTJVVTRVJUcnVzdCBSU0EgQ2VydGlmaWNhdGlvbiBBdXRob3JpdHkwHhcNMTgx MTAyMDAwMDAwWhcNMzAxMjMxMjM1OTU5WjCBjzELMAkGA1UEBhMCR0IxGzAZBgNV BAgTEkdyZWF0ZXIgTWFuY2hlc3RlcjEQMA4GA1UEBxMHU2FsZm9yZDEYMBYGA1UE ChMPU2VjdGlnbyBMaW1pdGVkMTcwNQYDVQQDEy5TZWN0aWdvIFJTQSBEb21haW4g VmFsaWRhdGlvbiBTZWN1cmUgU2VydmVyIENBMIIBIjANBgkqhkiG9w0BAQEFAAOC AQ8AMIIBCgKCAQEA1nMz1tc8INAA0hdFuNY+B6I/x0HuMjDJsGz99J/LEpgPLT+N TQEMgg8Xf2Iu6bhIefsWg06t1zIlk7cHv7lQP6lMw0Aq6Tn/2YHKHxYyQdqAJrkj eocgHuP/IJo8lURvh3UGkEC0MpMWCRAIIz7S3YcPb11RFGoKacVPAXJpz9OTTG0E oKMbgn6xmrntxZ7FN3ifmgg0+1YuWMQJDgZkW7w33PGfKGioVrCSo1yfu4iYCBsk Haswha6vsC6eep3BwEIc4gLw6uBK0u+QDrTBQBbwb4VCSmT3pDCg/r8uoydajotY uK3DGReEY+1vVv2Dy2A0xHS+5p3b4eTlygxfFQIDAQABo4IBbjCCAWowHwYDVR0j 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