Creating Optional Parameter in Powershell Function

I'm attempting to create a function with an optional parameter (called a switch?) that will export the results to a .csv.  Below is what I've managed to cobble together and it mostly works. If I don't specify -ExportCsv, the Export-Csv command requests a path be input. If I DO specify -ExportCsv, then it just runs and outputs the results on screen. I'd like to run the Get-DisabledAccounts function and it display on screen.  If the -ExportCsv parameter is specified, I want it to redirect output to a .csv file and bring up the save-file dialog. Where am I going wrong here?

Function Get-FileName
{ PARAM (
 $InitialDirectory
 )
  [System.Reflection.Assembly]::LoadWithPartialName("System.windows.forms") | Out-Null
  $SaveFileDialog = New-Object System.Windows.Forms.SaveFileDialog
  $SaveFileDialog.initialDirectory = $initialDirectory
  $SaveFileDialog.filter = "Comma Separated Values (*.csv)|*.csv|All Files (*.*)|(*.*)"
  $SaveFileDialog.ShowDialog() | Out-Null
  $SaveFileDialog.filename
  $InitialDirectory= "$env:UserProfile\Desktop"
}

Function Get-DisabledAccounts
{ PARAM (
 [string]$SearchBase="OU=Unicorn,DC=Squatty,DC=Potty",
 [string]$LastLogonDate=(Get-Date).AddYears(-1).ToString('MM/dd/yyy'),
 [switch]$ExportCsv=(Export-Csv -NoTypeInformation -Encoding UTF8 -Path Get-FileName)
 )
       Get-ADUser -SearchBase "$SearchBase" -Filter * -Properties sAMAccountName, LastLogonDate, HomeDirectory | Where-Object {$_.Enabled -eq "$null" -and $_.LastLogonDate -lt "$LastLogonDate"} | Select Name, sAMAccountName, LastLogonDate, HomeDirectory
}