Create New SSL CSR request file | Exchange Powershell 

This is a step by step quickest and easiest way to create a mail server SSL certificate request through Exchange Administrative Shell.
Script is below:
Generate New Certificate Request
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$txtrequest = New-ExchangeCertificate -GenerateRequest -FriendlyName CERTNAME -SubjectName "C=Seychelles,S=Mahe,L=Victoria,O=yourdomain.com,OU=IT,CN=mail.yourdomain.com" -DomainName mail.yourdomain.com,mail2.yourdomain.com,autodiscover.yourdomain.com -KeySize 2048 -PrivateKeyExportable $True
Import Certificate
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[System.IO.File]::WriteAllBytes('\\MAILSERVER\CSRFOLDER\MAIL4CERT.req', [System.Text.Encoding]::Unicode.GetBytes($txtrequest))