Add a function and put that in your worksheet IF-statement, like:
Public Function cellFunctionFirst(cellText As String) As String
Call thenModule(cellText)
cellFunctionFirst = cellText
End Function
Private Sub thenModule(cellText As String)
cellText = cellText & " text string added by macro."
End Sub
From the function call the module and do the work there.
Only thing is, that a function always gives back a value and you might not
want that. So then it's better to add a worksheet event that keeps listening
and launches another more macro (or not) after your function changed a cell
with a certain result.