To capture the text to a variable, you could use something as simple as a Prompt for User Input
but that's not going to show you a textarea, just a text input line. Next up would be a Custom HTML Prompt
to capture the text.
Or you could just take a look at Napkin v0.9k Text, Drawing & Photo Macro. It does a bit more than you need but also shows you how to append text tagged by the data and a slug to a running text file.