How to retrieve a users UPN suffix as a string - powershell

Question

I'm trying to get a specific AD User and change their UPN, but not their UPN suffix.
As you can see at the moment I have to manually enter their current UPN suffix which is a bit pointless since you have to go into AD to find that anyway, is there some string like $_.UPNSuffix that will call the user's current Suffix?
$container = "OU=MyOU,DC=MyDomain,DC=local"
$Filter = Read-Host -Prompt "Enter users Username/P-number"
$UPNSuffix = Read-Host -Prompt "Enter users current UPN Suffix"
$users = Get-ADUser -Filter "UserPrincipalName -like '$Filter*'" -SearchBase $container
Foreach ($user in $users)
{
$newFQDN = $user.GivenName + "." + $user.Surname
$NewDN = $user.GivenName + " " + $user.Surname
Set-ADUser -Identity $user -UserPrincipalName $newFQDN#$UPNSuffix -SamAccountName $newFQDN
Write-Host "User's UPN is now $newFQDN#$UPNSuffix"
}

Answer 1

You can get the UPN components by splitting on the # sign.
I would be doing something along the lines of:
$container = "OU=MyOU,DC=MyDomain,DC=local"
$Filter = Read-Host -Prompt "Enter users Username/P-number"
$users = Get-ADUser -Filter "UserPrincipalName -like '$Filter#*'" -SearchBase $container
Foreach ($user in $users)
{
$null, $UPNSuffix = $user.UserPrincipalName -split '#' # Dump the first part, store the 2nd
$newFQDN = $user.GivenName + "." + $user.Surname
$NewDN = $user.GivenName + " " + $user.Surname
Set-ADUser -Identity $user -UserPrincipalName "$newFQDN#$UPNSuffix" -SamAccountName $newFQDN
Write-Host "User's UPN is now $newFQDN#$UPNSuffix"
}

Answer 2

From a quick Google it doesn't seem that there is a dedicated field for the Suffix, but I figure you could get the UserPrincipalName property and then just split on the # and grab the second element of the split:
$UPN = (Get-ADUser -Identity $user -Property UserPrincipalName).UserPrincipalName
If ($UPN) {
$UPNSuffix = ($UPN -Split '#')[1]
} Else {
Write-Warning "Failed to get UserPrincipalName for $User"
}
Note: this is untested code.

Answer 3

It's possible to get the UPN suffixes from the uPNSuffixes field in the Partitions object who's located at :
CN=Partitions,CN=Configuration,DC=xxxxx,DC=com
Thanks from this post who provide an example in C# :
List all UPN Suffixes from Active Directory
I don't know how to implement that in powershell but in PHP, it's pretty simple :
ldap_read($ldapConnection, "CN=Partitions,CN=Configuration,DC=xxxxx,DC=com", "(objectclass=*)", array("*");
Maybe with Get-UserPrincipalNamesSuffix :
https://learn.microsoft.com/en-us/powershell/module/exchange/active-directory/get-userprincipalnamessuffix?view=exchange-ps
Hope this helps someone !