VBA check inputbox value origin

Question

I have a piece of code that asks the user to input text to write on my sheet, but, I want to do separate things when the user types plain text (simply typing the desired text) or the user references a sheet cell to get its value from as a input.

What can I do?

Dim mensagem As Variant
mensagem = Application.InputBox("Qual o motivo da pendência?", "O protocolo geral precisa saber...")
If Not (mensagem = False Or mensagem = vbNullString) Then
    If xxxx Then '(1) reference cell
        ActiveCell.EntireRow.Columns(17).Value = mensagem
        'do stuff
    ElseIf yyyyyy Then '(2) plain text
        ActiveCell.EntireRow.Columns(17).Value = mensagem
        ' do different stuff
    Else
        Exit Sub
    End If
End If
ActiveCell.Offset(1, 0).Select

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