Error Message Pop Up Alert

I wanted to make a pop up alert message appear whenever the formula in a cell gives a particular output.

Before posting this i tried looking for similar posts before, but the ones found do not solve my problem. There it asks me to write the following code in VBA,

Private Sub Worksheet_Change(ByVal Target As Range)
Dim MyRange As String

MyRange = "A1"

If Me.Range(MyRange).Value = "Have Meeting" Then
MsgBox "Have Meeting"
End If
End Sub

But my doing so, I can not use any formula in the cell A1 or any other cell i give reference as MyRange. I will always have to physically type "Have Meeting" or any other word assigned in the above.