You can add 3rd party code, such as Google Analytics or Chat,  by selecting Customize Website then Advanced then Custom Code. You will see the fields to add code either inside the head tag or footer (just before closing body tag).

If you are adding javascript, you can insert it in either head or footer areas. If you are adding custom CSS then it should go in the head field.

Make sure that you are certain as to the code you are adding as this can break your website if not done correctly.