VBA Code to throw prompt - vba

Question

Need your help on modifying the code or code that will help me do complete my task.
Its a protected workbook with VBA. I have a drop down on sheet1 in B18,D20,K11 and M46 and list of values in each drop down's.
When user changes the values in drop-down B18 and D20, I want to throw a prompt with "ok" and "cancel". User selects "ok" it should run a Module 1 ( which renames sheet).
Two things I am failing to achieve.
1. I don't want to throw prompt if user changes any other drop down values apart from 2 cells mentioned above,.
2. When clicked "OK" it should run module 1.
Please suggest if you have better code
Private Sub Worksheet_Change(ByVal Target As Range)
Dim rNg As Range
Set rNg = Range("B18", "D20")
MsgBox "Please click Calculate Button", vbOKOnly, "Calculate button"
End Sub

Answer 1

Try the code below.
(don't understand why you want to check if the user pressed "OK" since you are using a MsgBox with vbOKOnly and he has no other option)
Private Sub Worksheet_Change(ByVal Target As Range)
Dim rNg As Range
Dim IntersectRange As Range
Dim message As Integer
' modify here to the Range you need monitored
Set rNg = Range("B18", "D20")
Set IntersectRange = Intersect(Target, rNg)
If Not IntersectRange Is Nothing Then
message = MsgBox("Please click Calculate Button", vbOKOnly, "Calculate button")
If message = vbOK Then
Call Module1 ' or your Module / Function name
End If
Else
'Do Nothing if outside range
End If
End Sub

Answer 2

I am not saying this is the wright method, but you could use a UserForm to create custom prompts. And this is the way i do it, because for me it gives a better control on what i need to do.
Suppose if you create a userform with ok and cancel button, then you trigger the userform on the dropdown value change,
Sub DropDown1_Change()
UserForm1.Show
End Sub
Then write the codes for ok and cancel button clicks
Private Sub Ok_Click()
UserForm1.Hide
MsgBox "clicked ok"
End Sub
Private Sub cancel_Click()
UserForm1.Hide
MsgBox "clicked cancel"
End Sub