Negative experience? I did not have any particular negative experience, it was somewhat tiresome, and I just eventually decided it was not worth the time.
The bot was made using Selenium (https://www.selenium.dev), that's the most important piece of information I can give you. The code itself is lost somewhere in the memory of my home PC, and it does not work anymore anyway.
The idea is simple: you make a database of forums you want to go over, you keep there the links to the advertisement topics and some information about how to log in (because there are different scripts of PR-login circulating on mybb, so you need to keep it in mind). Then you make a bot using Selenium in any programming language gf your choice. The bot has to do several things in an exact order:
- go to the forum it's advertising
- log in
- go the advertisement topic
- copy the code from there and store it somewhere
- open a random forum from the database
- log in
- go to the advertisement topic
- copy the code from there
- go back to the advertised forum
- put the code there
- wait for the post to submit and get the link
- post the saved code and the link to the random form
- close the random forum
  -- repeat

The main trick here is that you have to constantly update the database: the advertisement links change, old forums die, new forums appear. From time to time, there are problems with loading something or a new PR-login script. Some forums just refused to load with Selenium, and I still don't know why. Overall, it would take a lot of patience to maintain this bot, but it is doable.