comma separator in textBox field

Question

I am new to .net

All i have is a text box that is not accepting "Comma's" ex. when I enter multile accountIds like Account1,Account2,Account3

Below is the code

<asp:TextBox ID="txtAccountID" runat="server" ClientIDMode="Static" CssClass="txtSearch InputMask"

Width="340px"></asp:TextBox>

I made the ClientID to AutoID/Predictable and inherit but no luck

How can I make this text box accept Comma's