Hi Rami,
Thanks for contacting Syncfusion support.
Through HtmlAttributes property, we will add the attributes to wrapper of input element instead input element except the following attributes disabled, required and readOnly. So we suggest you to add the attributes to element directly by getting it from DropDownList object if it need.
The following JS Playground sample will show case the demo for the requirement.
http://jsplayground.syncfusion.com/nl1qrbq5
Please let me know if any concerns.
Regards,
Arun P