I do not know what the feature is called. Perhaps someone else does.
If you read
this[
^] page, particularly the
Type or write notes on a page section. The first paragraph give you lots of clues as to what they do.
You have to do the same. When your user clicks anywhere in your text region you have to create a text entry control of some sort that will expand to fit whatever text they type. If you are really clever you might also make it so that they can tilt the text after entry is finished. A lot of work though.
Inside other controls this sort of functionality is called
in place editing and you might want to try that as a search phrase to get you started.