Thanks Jetze
Claus sorted it out for me. Simple double quote is all I was missing. I am working on creating a GUI for it now.
$UPN = Read-Host "User Principal Name"
New-Mailbox -Name $FullName -Alias $FullName -Shared -OrganizationalUnit 'Corp.Company.AD/Users' -PrimarySmtpAddress $Email -RetentionPolicy 'Default Retention Policy' -UserPrincipalName "$UPN@corp.company.ad" -SamAccountName $FullName -FirstName $FullName -Database 'DB2' -whatif