Go to "Settings" (on the top of the website builder);
Go to "Settings" menu items there and switch to "Scripts" tab;
Insert your code into one of 3 available fields:
"Before " – inserts code in the ending of page/website section
"Before " – inserts code in the ending of page/website section
"After " – inserts code in the beginning of page/website section
The script can be added either for whole website (all pages) or for exact page.