Hi Steve,
Thank you for your interest in Syncfusion Products.
Please refer the sample below which illustrates the TextBox placed inside UpdatePanel and when MenuItems are selected, the ItemSelect event get raised. The ControlID should be specified by the Menu control's ID and EventName is not neccessary for triggering.
http://websamples.syncfusion.com/samples/Tools.Web/6.4.0.15/F79097_Menu-UpdatePanel/main.htmPlease let me know if this sample helps you.
Thanks,
Rekha