call the button click event programmically

I have customised the billing.aspx page. I now need to programmically call the click event of the continue image button to continue to the next page. I can't fire the click event. Does any one know the click event name of the continue button?

I've tried using this line to call the event

this.btnNext_Click(this, new EventArgs());

or

this.btnNext_Clicked(this, new EventArgs());