Not in the code behind - you'd have to use Javascript. In this case, the
web page is wholly on the client, and there is no communication with the
server -- so how would a LostFocus event fire in the code running on your
server?
The explanations provided are correct. However, it is possible to make this
work. Trap the onblur event or some similar event clientside using
javascript, then forcibly submit the form with a hidden field indicating
what just happen. In your page load event, you retrieve this hidden field
and call an appropriate function. This creates a *manual onlostfocus event
handler.