How to Invoke Required Validator Explicitly

[sinureddy]

I am working on ASP.Net2.0

I have a TextBox, a associated RequiredFieldValidator and a
LookupButton.
I do have a Save Button.

Without entering anything in TextBox I clicked Save Button. Required
Field Validator is visible now.
I click on Lookup Button and a new window is popped up from where I
select a value and click the OK Button present in Popup. Clicking the
OK Button in the Popup closes the Popup and the fills the Text
box(present in the main window) withe value selected in Popup window.
The focus is put on TextBox just before TextBox is filled from Popup.
But the required Validator is still there. How should I tackle this?


Thanks
Srinivas Reddy Admala