code to bring up the save as window

You're title is misleading. Can you please edit it as per forum rules below

Something like

Dim Ans As String
    Ans = MsgBox("Save changes ?", _
        vbYesNo, "Save File")
 
    If Ans = vbNo Then Exit Sub ' the macro ends if the user selects the CANCEL-button
    If Ans = vbYes Then Application.GetSaveAsFilename

VBA Noob

Maybe like this. Code goes in ThisWorkbook module.

Private Sub Workbook_Open()
    Dim sFile   As String
    
    With ThisWorkbook
        .Saved = False
    
        If MsgBox("Save as new file?", vbYesNo) = vbNo Then .Close SaveChanges:=False
    
        sFile = Application.GetSaveAsFilename
        If sFile = "False" Then .Close SaveChanges:=False

        .SaveAs sFile
        If Not .Saved Then .Close SaveChanges:=False
    End With
End Sub