CheckBoxControl.InputAttributes

default.aspx

<form id="form1" runat="server">
	<div>
		<asp:checkbox runat="server" id="checkbox" oninit="checkbox_Init" />
		<asp:button runat="server" />
   	</div>
</form>

default.aspx.cs

private void Page_Load(object sender, EventArgs e)
{
	if (!IsPostBack) {
		checkbox.InputAttributes.Add("x-data", "!");
	}
}

private void checkbox_Init(object sender, EventArgs e)
{
	var _ = checkbox.InputAttributes;
}

MyCheckbox.cs

public class MyCheckBox : CheckBox
{
	protected override void OnInit(EventArgs e)
	{
		var _ = this.InputAttributes;
		base.OnInit(e);
	}
}