Hi jessica,
You can use javascript to close the window.
If you want to get the confirmation before close the browser then you can use the window.close() method.
<input id="btnClose" value="Close" type="button" onclick="window.close();" />
If you do not want to show any confirm before closing the browser then use the following.
<input id="btnClose" value="Close" type="button" onclick="window.opener='x';window.open('','_parent','');window.close();" />