asp.net - Issue with in texbox

Question

I want to avoid, and get rid of, any blank space that is showing up like this in my textbox " " How can I not show this blank space when the page loads? I am using a popup model extender so when it loads I get the blank space which causes an error when I try to save the data.

Here is my textbox:

<tr>
    <td>
        <asp:Label ID="Label24" runat="server" Text="BO_Start"></asp:Label>
    </td>
    <td>
        <asp:TextBox ID="Start" runat="server"></asp:TextBox>
    </td>
</tr>