Code insertion right after head tag

I want to give a try to adsense, I have a host adsense account for my youtube channels.
I tried to upgrade and than aded their script code in my html head tag via a plugin. I done it about one week ago. I have not received any email from adsense about approval or disapproval.
I am thinking that html code is not properly inserted.
I have searched google for edit my html but not found any accurate ans. In wordpress theme editor, there are only php files.
Now I have install and activate this plugin, after this have I need to add that code right after the head???