question about contact us form

hello. madam ,thank you for replying
i don't understand what you say,which property should i choose in the behavior tab of validator to show an alert box.i am attaching a simple file could you please put an alert box if the textbox
left empty?

alert box was being generated if it is empty is it because that is the behavior of requiredfieldvalidator in VS

can't find the downloadable file on that link please have it uploded on this thread..thanks

i have already atach a link it is a c sharp cornerpage and a download file is inside that link.although
i have attach a file with this text,please check this and guide me.thank you