those things can be called footprints
find a site (or multiple) that allow comments. see what possible footprints they have.
for example 1 footprint may be "powered by wordpress" + "leave a comment". put it exactly like that into google query (including the quotations and plus sign)
scrapebox software has a powerful module that can scrape+submit thousands of comments in a few seconds. there's other automated tools too.
but it's not that easy. most webmasters filter or don't allow comments. some forms require google recaptcha. and many of the ones that allow comments already have 1000+ spammy outbound links on it.
another footprint may be "powered by disqus". keep in mind disqus has their own anti-spam modules baked in. if you can crack it, and get your comments to stick, you'll make a lot of money very fast.
back to the topic, if you hit the search bar you will find other members who shared footprints like this. it used to be a very hot method years ago. it still can be with various twists. contact form spam may be a good search phrase to look into.