According to this answer here: https://www.facebook.com/help/community/question/?id=10200354561858276

you can place it inside the  element but FB recommends to keep it at the beginning of  or in the  as the conversion will be then counted even if the page does not load fully or the user closes the page.

*** And facebook want to count it as conversion but if the user closes the page before it loads, from a real perspective this is not a conversion. This means that it should not be counted at all. I think that this conversion code should be put at the end of the body. Last possible tag