Refer the below sample.
HTML
<div align="center">
<asp:ScriptManager runat="server" />
<div style="text-align: center; margin-bottom: 10px; font-size: 20px; color: brown;">
<asp:Label ID="lblmsg" runat="server"></asp:Label></div>
<asp:GridView ID="GridView1" runat="server" HeaderStyle-BackColor="#3AC0F2" HeaderStyle-ForeColor="White"
AutoGenerateColumns="false" OnSelectedIndexChanged="OnSelectedIndexChanged">
<Columns>
<asp:BoundField DataField="Emp1" HeaderText="Emp1" />
<asp:BoundField DataField="Emp2" HeaderText="Emp2" />
<asp:BoundField DataField="Emp3" HeaderText="Emp3" />
<asp:ButtonField Text="Select" CommandName="Select" />
</Columns>
</asp:GridView>
<asp:LinkButton Text="" ID="lnkFake" runat="server" />
<cc1:ModalPopupExtender ID="mpe" runat="server" PopupControlID="pnlPopup" TargetControlID="lnkFake"
CancelControlID="btnClose" BackgroundCssClass="modalBackground">
</cc1:ModalPopupExtender>
<asp:Panel ID="pnlPopup" runat="server" CssClass="modalPopup" Style="display: none">
<div class="header">
Details
</div>
<div class="body">
<asp:TextBox ID="TextBoxName" runat="server" placeholder="Your Name" Style="width: 126px;
float: left; height: 25px; border: 1px solid; border-color: #c4c4c4 #d1d1d1 #d4d4d4;
border-radius: 2px; outline: 2px solid #eff4f7; color: #404040; padding: 0 10px;"></asp:TextBox>
<asp:RequiredFieldValidator ID="RequiredFieldValidator1" runat="server" ValidationGroup="A"
ControlToValidate="TextBoxName" ErrorMessage="Required field"></asp:RequiredFieldValidator>
<br />
<asp:TextBox ID="TextBoxEmail" runat="server" placeholder="Email Address" Style="width: 126px;
float: left; height: 25px; border: 1px solid; border-color: #c4c4c4 #d1d1d1 #d4d4d4;
border-radius: 2px; outline: 2px solid #eff4f7; color: #404040; padding: 0 10px;"></asp:TextBox>
<asp:RegularExpressionValidator ID="RegularExpressionValidator1" runat="server" ControlToValidate="TextBoxEmail"
ErrorMessage="Format Email Error" ValidationGroup="A" ValidationExpression="\w+([-+.']\w+)*@\w+([-.]\w+)*\.\w+([-.]\w+)*"></asp:RegularExpressionValidator>
<br />
<asp:TextBox ID="TextBoxNots" runat="server" placeholder="Nots" TextMode="MultiLine"
Rows="4" Style="width: 282px; float: left; margin-left: 7px; border: 1px solid;
border-color: #c4c4c4 #d1d1d1 #d4d4d4; border-radius: 2px; outline: 2px solid #eff4f7;
color: #404040; padding: 0 10px;"></asp:TextBox><br />
<asp:Button ID="Button1" runat="server" Text="Rigester" CommandName="Select" Style="width: 267px;
float: left; margin-left: 7px; border-radius: 2px; height: 29px; border: 1px solid;
margin-top: 3px;" OnClick="SendEmail" ValidationGroup="A" />
</div>
<div class="footer" align="right">
<asp:Button ID="btnClose" runat="server" Text="Close" CssClass="button" />
</div>
</asp:Panel>
</div>
Code
Protected Sub Page_Load(sender As Object, e As EventArgs)
If Not Me.IsPostBack Then
Dim dt As New DataTable()
dt.Columns.AddRange(New DataColumn(2) {New DataColumn("Emp1"), New DataColumn("Emp2"), New DataColumn("Emp3")})
dt.Rows.Add("A1", "A2", "A3")
dt.Rows.Add("B1", "B2", "B3")
dt.Rows.Add("C1", "C2", "C3")
GridView1.DataSource = dt
GridView1.DataBind()
End If
End Sub
Protected Sub OnSelectedIndexChanged(sender As Object, e As EventArgs)
mpe.Show()
End Sub
Protected Sub SendEmail(sender As Object, e As EventArgs)
Dim emp1 As String = GridView1.SelectedRow.Cells(0).Text
Dim emp2 As String = GridView1.SelectedRow.Cells(1).Text
Dim emp3 As String = GridView1.SelectedRow.Cells(2).Text
Dim Name As String = TextBoxName.Text
Dim TextEmail As String = TextBoxEmail.Text
Dim TextNots As String = TextBoxNots.Text
' Code for sending mail.
mpe.Hide()
End Sub