How to add review comments on code in master branch

Question

There is a repository on github, I have to review that. But all the code is there in the master branch. So I am unable to create any pull request for it as there is only one branch master and so not able to put any review comments on the code. Is there a way I can put review comments without opening a pull requests?
Or some alternative way to open a Pull request in this case?

I tried creating a pull request, but since there is only one branch its not possible. When I open a file directly on the github repository, I dont see any option to put comments.
An alternative I could think of is to create an empty branch and then create a PR. But could there be a better way?

Answer 1

0. So, you are on a master branch on GitHub.com
1. Open the file you want to review
2. Find the line where you want to leave a comment
3. Press "Blame"
4. Find out which commit has created/updated this line last time. Open this commit
5. When you opened the target commit from "blame", you are able to leave comments and mention author with @