Personally I despise the InputBox function but you could do something like
Dim fileName As String
Do
filename = InputBox("Input a filename:", "File Lister", "my movies", 200, 300)
Loop Until fileName <> ""
although I would use something more user-friendly like
Dim sf As New SaveFileDialog
sf.Title = "Input a filename"
Dim fileName As String
Do
sf.InitialDirectory = Application.StartupPath
sf.ShowDialog()
fileName = sf.FileName
Loop Until fileName <> ""