[ndoktoruser]

Thanks for the hint, Replicant!

However, maybe I didn't explain very well what I want to do.
I do not want to change the content submitted by the form (/js/ckeditor-rollup-519.js). Is this the best option?

I imagined the following:
1) receive the content submitted by the form;
(probably some API extension would do it, with no changes to the JS)
2) validate whether the check box for anonymous post was checked or not;
3) if so, change the data which will be saved for the post (in the table node);
3.1) the current API saves the "userid" and "authorname" in the table "node", so I would set values for an anonymous post (0, "Guest");
3.2) at the same time, I would store the real "userid", "authorname" and "nodeid" in a new table created by the plugin (this information might be important for moderation).

I believe that this would be the simplest way, because I wouldn't change any existing code.
And when the topic is displayed, the current API would fetch the correct "anonymous" id and user from the table node.

PS: I changed the initial post to add the code for the hook.
PS2: the feature would have the same functionality as if we allow "non registered users" post in the community, but only for "registered users". And would add the log in the new table. So, technically vB already does that, I just don't know where.