Add onblur to htmlattributes
<%=Html.TextBox( "ChooseOptions.AddCount" + order.ID, (order.Count > 0) ? AddCount.ToString() : "", new { @class = "{number: true} small-input", onblur = "alert('fired')" } ) %>
Or better add it using jQuery
$('#ChooseOptions_AddCount' + id).onblur(function() { alert('fired'); });
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