Do you have a link to a guide that can let me customise the create new topic dialog? By for example adding more like adding things to the input fields that are already there?
To modify the composer or as you called it the new topic dialog, you’d need to write a plugin. You can see all the plugin guides here: #plugin-guides, but I believe the specific one you want to look at is Beginner’s Guide to Creating Discourse Plugins Part 2: Plugin Outlets - howto / developers - Discourse Meta.
That said, it would help you if you went through all of them from the first one (part 1) to the last one (part 7).