Getting text in textbox(in webbrowser control)

Question

Hi..
I am using a webbrowser control in my form program and i am displaying an html page. There is 1 textbox in html page and id  is "textbox".  When someone write something in textbox, i want to use this text and save it to db. It will be work like this; somebody is writing somethings in textbox(in html page) and when press the button(form button), i want to save this text.

I suppose I should use innerhtml but actually i dont know how use it?