Setting ShowHeaderCheckbox TRUE dynamically in client side.

[Elyfran Vaz]

How can i do for set true to attribute "ShowHeaderCheckbox" dynamically in client side?

<ext:GridPanel ID="Grid1" runat="server" StoreID="Store1" Title="Company List" Collapsible="true"
        Width="600" Height="350">
<SelectionModel>
    <ext:CheckboxSelectionModel ID="sltModel" runat="server" Mode="Multi" ShowHeaderCheckbox="False"/>
</SelectionModel>
</ext:GridPanel>

thx