Hi learningmrs,

Refer below code.

C#

protected void OnSave(object sender, EventArgs e)
{
    string url = Request.Url.AbsoluteUri;
    string script = "window.onload = function(){ alert('Details saved sucessfully.');";
    script += "window.location = '";
    script += url;
    script += "'; }";
    ClientScript.RegisterStartupScript(this.GetType(), "Redirect", script, true);
}

VB.Net

Protected Sub OnSave(ByVal sender As Object, ByVal e As EventArgs)
    Dim url As String = Request.Url.AbsoluteUri
    Dim script As String = "window.onload = function(){ alert('Details saved sucessfully.');"
    script += "window.location = '"
    script += url
    script += "'; }"
    ClientScript.RegisterStartupScript(Me.GetType(), "Redirect", script, True)
End Sub

For more details refer below article.

Call JavaScript function using ClientScript before Response.Redirect in ASP.Net