I have a onclick event that is like:
public void OnMyButton_Click(object sender, EventArgs e)
How can I call this from within pageload?
Event handlers are just normal methods - you can call them from anywhere within the class they're defined in - so Dan's answer is technically correct, although I feel he leaves out a bit of best-practice advice.
Instead of calling your event handler from your load event, you should move the code in your event handler into another method, and then call that method from within both your click event handler, and your page load method.