Really, Site wide code injection isn’t meant for ads as it will add ads to top of site and into the footer of site. Both are not a good place to place your ads.
Injecting into individual post is such stupid thing, I can’t believe you guys are recommending it.
I want to add ads manually I know I have to put it into post.hbs file but I don’t know where to put the code to display ads where I wanted to. So if you can guide a bit in that direction that would be helpful.