script type="text/javascript">
        function JumpToNextTextBox(currentTxtBox, nextTextBoxID) {
            if (currentTxtBox.value.length >= 5) {
                document.getElementById(nextTextBoxID).focus();
            }
        }
    /script>

  License Key:
            asp:TextBox ID="Number1" runat="server" onkeyup="JumpToNextTextBox(this, 'Number2')" Width="65">/asp:TextBox>
            -
        asp:TextBox ID="Number2" runat="server" onkeyup="JumpToNextTextBox(this, 'Number3')" Width="65">/asp:TextBox>
            -
        asp:TextBox ID="Number3" runat="server" onkeyup="JumpToNextTextBox(this, 'Number4')" Width="65">/asp:TextBox>
            -
        asp:TextBox ID="Number4" runat="server" onkeyup="JumpToNextTextBox(this, 'Number5')" Width="65">/asp:TextBox>
            -
        asp:TextBox ID="Number5" runat="server" MaxLength ="5" Width="65">/asp:TextBox>