If you upgraded from BPS free to BPS Pro then your Custom Code should already exist and you don’t need to do anything except for run the BPS Pro Setup Wizard. If you are talking about Bonus Custom Code then you do not need to add any Bonus Custom Code unless you want additional specific security protection for your website. If you do want to add Bonus Custom Code then I recommend that you only add these below:
Mime Sniffing, Data Sniffing, Content Sniffing, Drive-by Download Attack Protection
https://forum.ait-pro.com/forums/topic/mime-sniffing-data-sniffing-content-sniffing-drive-by-download-attack-protection/
WordPress Author Enumeration Bot Probe Protection
https://forum.ait-pro.com/forums/topic/wordpress-author-enumeration-bot-probe-protection-author-id-user-id/
Block other websites from displaying your website pages or Feeds in iFrames, Clickjacking Protection
https://forum.ait-pro.com/forums/topic/rssing-com-good-or-bad/
Speed Boost Cache Code (if you are not already using a caching plugin that adds/creates Browser caching htaccess code)
https://forum.ait-pro.com/forums/topic/htaccess-caching-code-speed-boost-cache-code/