Pre-filled editable text box

I want to have an editable text box that is pre-filled with text we are suggesting they use for submitting to a politician. So our text is there, and they can either use that, parts of it, or completely change it.

The screenshot I'm providing is from an email that I received from someone else, but that's what I'm going for.

Hi,

You can use the Default Value feature. Add the Text you want and let the user edit it if they want. Please check the screencast guide below: