Hi Ramco1917,

Please refer below sample.

HTML

Default

<asp:LinkButton ID="lnkbtnAdd" runat="server" PostBackUrl="~/ActionType.aspx?value=A"
    class="btn bg-info btn-sm ml-3"><i class="icon-plus3 mr-2"></i>Add</asp:LinkButton>

Code

ActionType

C#

protected void Page_Load(object sender, EventArgs e)
{
    if (string.IsNullOrEmpty(Request.QueryString["value"]))
    {
        ClientScript.RegisterStartupScript(this.GetType(), "alert", "alert('Required!');", true);
    }
}

VB.Net

Protected Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs) Handles Me.Load
    If String.IsNullOrEmpty(Request.QueryString("value")) Then
        ClientScript.RegisterStartupScript(Me.GetType(), "alert", "alert('Required!');", True)
    End If
End Sub

Screenshot