In the report feature, there's extra details where you can tell us exactly what needs to be changed.

Ok, but I have to choose one of the options. There is only "incorrect content", which doesn't actually apply since I want to add something.

If you want to change some content, then I assume you are not entirely happy with the current information. Therefore "incorrect content" would be a a natural choice, if you think you can help improve the data. If the current information is fine, then there is no need to change.