Refer the below link and chage as per your requirement.

Automatically generate AlphaNumeric Unique number in code behind VB.Net

Else you have to do like below.

HTML

<div>
    <asp:Button Text="New CID" runat="server" OnClick="NewCid" />
    <br />
    <asp:Label ID="lblnewCId" runat="server" />
</div>

Code

C#

protected void NewCid(object sender, EventArgs e)
{
    lblnewCId.Text = GenrateNewCId();
}

private string GenrateNewCId()
{
    string newCId = string.Empty;
    if (Session["CID"] == null)
    {
        Session["CID"] = DateTime.Now.ToString("yyMM") + "0000";
        newCId = Session["CID"].ToString();
    }
    else
    {
        string cId = Session["CID"].ToString();
        int previousCId = int.Parse(cId.Substring(4));
        previousCId++;
        newCId = DateTime.Now.ToString("yyMM") + (previousCId).ToString().PadLeft(4, '0');
        Session["CID"] = newCId;
    }
    return newCId;
}

VB.Net

    Protected Sub NewCid(ByVal sender As Object, ByVal e As EventArgs)
        lblnewCId.Text = GenrateNewCId()
    End Sub

    Private Function GenrateNewCId() As String
        Dim newCId As String = String.Empty

        If Session("CID") Is Nothing Then
            Session("CID") = DateTime.Now.ToString("yyMM") & "0000"
            newCId = Session("CID").ToString()
        Else
            Dim cId As String = Session("CID").ToString()
            Dim previousCId As Integer = Integer.Parse(cId.Substring(4))
            previousCId += 1
            newCId = DateTime.Now.ToString("yyMM") & (previousCId).ToString().PadLeft(4, "0"c)
            Session("CID") = newCId
        End If

        Return newCId
    End Function

Screenshot