I have been on other writing websites that have implemented Adsense - but it has to be done by the owner of the domain, in this case Steemit.
Also, if you look at the standard URL of each post, it is:
steemit.com/tag/username/post-title
so unless you always post in the same tag, your Adsense code won't work for all posts.
I think you may be able to insert Amazon code.