Loading default values of combobox?

I am trying to develop a website in php and I am dynamicaly loading a page into a <div> using its id with the help of Javascript.

That loaded page has a combo box and I want as the page load in <div> the first option of combo box should be displayed in text box on the same page. I tried onload option but it is not working as page is not dynamically loading instead its output is loading by javascript in <div>.

Loading default values of combobox?