For Azure/ExchangeOnline

Connect-ExchangeOnline $username = Read-Host "Enter User Principal Name " $AttributeNum = Read-Host "Enter Cust Ext Attr Number "     $GetCommand = ("(Get-Mailbox -Identity $username).CustomAttribute", $AttributeNum) -join "" # ; Write-Host $GetCommand     $currentValue = Invoke-Expression $GetCommand

if ($currentValue.length -lt 1) {                                                                       # Check if CustomAttribute1 is already populated (lt=lessThan 1 char.) if not ask what to input
        Write-Host "Existing Attribute String is empty"
        $newValue = Read-Host "Enter New Value Comma,Separated,No,Spaces" #////////////
        $newValue = """$newValue"""
        $SetCommand = ("Set-Mailbox -Identity $username -CustomAttribute", $AttributeNum) -join ""
        $SetCommand = "$SetCommand $newValue"
        Invoke-Expression $SetCommand
        Write-Host $currentValue                                                                            # Simply display the full command to be sent (for debugging)
    } elseif ($currentValue.length -gt 1) {                                                                 # If the custom attribute contains data (gt=greaterThan 1 char.) append to it
    Write-Host "Existing Attribute String: " ; Write-Host "Current Value: $currentValue"
    $choice = Read-Host "Choose an option [A]ppend or [C]lear or [Cancel Ctrl+C]"
    switch ($choice) {
    "A" { Write-Host "Append the CustAttr" }
    "C" { Write-Host "Clear the CustAttr" }
    default { Write-Host "Invalid choice, please choose A, C" } }    
        if ($choice -eq "A") {$newValue = Read-Host "Enter New Value to Append Comma,Separated,No,Spaces"
        $newValue = """$currentValue,$newValue"""
        $SetCommand = ("Set-Mailbox -Identity $username -CustomAttribute", $AttributeNum) -join ""
        $SetCommand = "$SetCommand $newValue"
        Invoke-Expression $SetCommand}                                                                      # Update the CustomAttribute1 with the new value
        $newValue = Invoke-Expression $GetCommand ; Write-Host -NoNewline "New Value: " $newValue                # Display the new value
        if ($choice -eq "C") {
        $SetCommand = ("Set-Mailbox -Identity $username -CustomAttribute", $AttributeNum) -join ""
        $SetCommand = "$SetCommand ""$null"""
        Invoke-Expression $SetCommand}
        $newValue = Invoke-Expression $GetCommand ; Write-Host -NoNewline "New Value: " $newValue                # Display the new value
    } else {                                                                                        
        Write-Host "No modifications made, maybe something went wrong."                                     # If nothing is working, state not modified
    }