that would go in your (child-)theme's functions.php or in a seperate small plugin. to be clear; this does not make autoptimize act on external webfonts, it just loads the fonts late(r) (the resulting HTML is placed in the footer instead of head). This will, one should be warned, lead to the page being rendered with native fonts (available in browser without download) after which the webfonts are downloaded and the text switches.
frank