New button (star) in Writer Field

I followed this tutorial (Block factory: Creating your own blocks collection) and replicated the content “box”. Works perfectly.

When I mark text there, a new and so far unknown icon appears in the small popup window with the different options (Bold, Italic, Strike, Underscore, Code, Link, E-Mail): a STAR. When this is clicked, the highlighted text appears with a yellow background and the <mark> tag is added to the output.

However, I am unable to get this star to appear on other Writer Fields. How do I activate it?

Here’s the simple blueprint:

text:
	label: Text
	type: writer
	placeholder: Enter some text…