How to Get value from zen component

I am new to zen pages, and trying to see how to get the value of a control.

Below is what I have

<combobox id="comboboxEdit" label="Select">
<option value="1" text="1" />
<option value="2" text="2" />
</combobox>

<button id="appointments" label=" " caption="Fetch" onclick="zenPage.Fetch();" hidden="false"/>

ClientMethod Fetch() [ Language = javascript ]
{

// how can I get the value of the controls here? can I do something like document.getElementById()?

}