Unfortunately, if you have a free website on neocities created after ~2022 you are not allowed to use external scripts, such as guestbooks, shoutboxes, status/mood widgets, and even hit counters.
The good news is that I found a way around that: you can use iframes. Specifially, you use the script you want to use on a different website not hosted on Neocities (instead hosted on e.g. github pages), in the center of an otherwise blank page with background: transparent; and then use this URL in an iframe on your neocities page. Here you can see an example of what an otherwise blank page with a widget may look like.
For guestbooks I recommend using smartgb (which is what I use for my guestbook iframe) because it is very customizable and looks good in an iframe. For a chatbox you can use cbox, because it works via an iframe.