FormAction how to setIcon()

Silverstripe Version:5.1

How do you set an icon on a FormAction using the setIcon() method?

I tried to change the ‘Go’ for the SiteSearch and was first of all chocked to see that the ‘Go’-button wasn’t actually a button by default but an input field. This is not really a11y-friendly.

I then tried to use the setIcon() method which was described to use the icon identifier and not the path. I tried a fontawesome icon and unicode, but no icon showed up.

Then I gave the setButtonContent() method a go and that worked with both.

Is this a bug or what did I need to do in order to get he setIcon() to work?
Could someone give me an example please?