The correct way to add a CSS class to a RadTextBox programmatically on the client is the following:
var tb = $find(......);
tb.get_styles().EnabledStyle += " conflict";
You may need to add the custom CSS class to different styles, e.g. HoverStyle, FocusedStyle, InvalidStyle, etc. In addition, updateCssClass() should be called always after modifying the RadTextBox styles collection on the client.
addCssClass is a method of the Sys.UI.Control type, not ours. It is not supported by RadTextBox, because the element of the control is hidden and not visible by the user.
the Telerik team