How to set a default text in a text area?

How can I add text to the section circled to the left of the box name? 

I'm not sure what are you trying to achieve, but there are two ways to do this, the first one is setting a default value, if this value is not changed this will be the value in your submissions, the second method is setting a hint example, known in HTML as placeholder text, this will be an example about the text that you need to be entered there, this value will not be submitted if this is not changed, it will only be empty. 

To set the default value you need to click on the field and the field's properties will be displayed on the top toolbar, click on the option Default Value and type there the value that you want: 

If you want to set a hint example then you only need to click on the option Hint Example: 

Hope this helps.