i want code for that
It does not work like this here.
Here is what is expected of enquirers:
1. TRY first
what you want to do! You may find that it's not that hard.
2. Formulate what was done by you that looks like an issue/not working.
Try them and tell if you face issues.
Members will be more than happy to help like this.
For now, you need to:
After page postback, in the submit event (if a button submitted the page), look for the text value and show a message in case value is nothing.
I would by far suggest option 1.
Try out and post specific issues if you have any.