set value of txtbox inside iframe of different domain.

Hi, guys

              I have my asp.net c# web page and a iframe in it. In the iframe i have a another website like www.gmail.com that is different domain. On my page i have a text box and button. when i click on button my text box value set to the gmail.com textbox. Means value pass to the different domain that is inside the iframe on my page. Please guide me.