Hi makumbi,

Please refer below sample.

Namespace

VB.Net

Imports System.Windows.Forms

Code

VB.Net

Protected Sub Page_Load(sender As Object, e As System.EventArgs) Handles Me.Load
    Dim value As String = "100"
    Dim result As Integer = MsgBox("You have enetered Less money by: " _
                        & value & Chr(10) & Chr(10) & _
                        "If this is a credit sale enter the account No of the Customer, Is it a credit Sale ?", vbYesNo, "SPOS")
    If result = 7 Then
        MsgBox("No clicked.")
        Exit Sub
    Else
        MsgBox("Yes clicked.")
        Exit Sub
    End If
End Sub

Screenshot

ASP.Net is a Client Server technology where the program runs on Server while Page is displayed on Client in a Browser, while MsgBox function is a Windows Forms function and hence when used in ASP.Net it displays Message Box on Server.

Thus in ASP.Net, the messages needs to be displayed with the help of JavaScript Confirm Message Box where JavaScript is a Client Side script which runs within Browser.

Refer below article.

Server Side (Code Behind) Yes No Confirmation Message Box in ASP.Net