As the title says

Not sure what you mean by Actionbar, is that like the swing percentage bar under the crosshair after you use an item?

You can try to run this command
title @s actionbar {"text":"This is only a test"}

I didn't even know that was a feature until now, I'll add it when I get the chance.

Yes its added at 1.11

Fixed in commit c435be9, will add title and subtitle when I get the chance.