I am using MindManager 2020 and I have a created a map and I have added a Text Box on the map background. I want to do some formatting to this Text Box (like adding a border, thickness and color) but I can't seem to find any formatting options. What am I missing here?
No, unfortunately you can only format the text itself, and not the box.
To set up a text box with a border you have to create a separate text box and a shape superimposed on each other, and group them. It seems better to start with the shape first; format that as appropriate then click inside and create the text box and edit the text, then right click on both of them and select Group (you may have to play with the order of the elements to access both of them).
Somewhat oddly you can label a shape and you can format this label, and place it above, next to, or below the shape - but not inside.
Or... maybe an easier way to get what you want.... Just add a floating topic and you can format that as you like. The only thing you have to be aware of is that you can move a floating topic to any place if you do shift-drag. If you do just drag, you may find it tries to connect to the nearest topic in your map.