Hello @sanyaman90,
If I understood you correctlly, you need to capture the value that user selects from the ListBox, so you could pass that value to your function and perform search. ListBox control has a property called SelectedItem, here is example in PowerShell not C#.
The problem with your code is that although your variable name is $ListBox, actual object is TextBox not ListBox
#TextBox Object Creation
$ListBox = New-Object System.Windows.Forms.TextBox
#ListBox Object Creation
$ListBox = New-Object System.Windows.Forms.ListBox
Hope that helps.