My checkbox in asp.net doesn't work with the codebehind file

I've got a checkbox, which should go to a method in the codebehind file. This is my aspx code:

<input name="fullscreen_chbx" type="checkbox" id="fullscreen_chbx"  onclick="fullscreen_chbx_Click" runat="server"/>

And this is my codebehind code. The code should work but it doesn't even execute the method.

protected void fullscreen_chbx_Click(object sender, EventArgs e)
{
    bool fullscreen = false;


    if (fullscreen == false)
    {
        InputSimulator.SimulateKeyPress(VirtualKeyCode.F11);
    }
    else
    {
        InputSimulator.SimulateKeyPress(VirtualKeyCode.F11);


    } 

There's always this error in the aspx file: runtime error in JavaScript: "fullscreen_chbx_Click" is undefined. Why is there a error with javascript?