Explicit vs. Implicit "login required" message for comments section

Let me explain:

In order to get more comments on a website's article, the best approach is:

  1. To let the user know beforehand that they have to sign in, in order to make a comment

  2. To simply let a comment box for the user to write in, along with a submit button, and when the user hits this button, a "login required" popup shows up giving the options to Sign In or to Sign Up

And if it is not too much to ask, could you explain the reason for your answer? Thanks.