How to add a Textbox to a map

1. In the map editor, under Custom HTML, there's an object called "Custom HTML Element".
2. Simply drag it on the map.
3. Now click it, so it's highlighted.
4. On the right side, within the objects properties, there's a propery HTML and a small arrow.
5. Click the arrow to reveal the HTML Before and After fields. Enter your HTML there, for example <b>Hello World</b>
6. Hit Save.

You're done :)

For more details, see also the article Adding a Textbox to a Map.

This works but it could be easier if there was a textbox map object.