Asp – How should I bind to the textbox’s events

I am populating a textbox (txtFileNature) with a value from a popup window. The textbox has an onfocus handler set to


The problem is that I want to do some calculations based on the value of txtFileNature and want to display the result below it. In which event can I can use my VB code. I can't use the textchanged event since I am not typing in the textbox.