It doesn’t look like there’s a plugin outlet to put the button exactly where you want:
Maybe you could settle for having it at topic-above-posts
.
Then you’d make a glimmer component that adds the button. Finally, you’d use api.renderInOutlet
to have that component show up there. Custom Components -- add button or text at any plugin outlet is an example of that.
So then you’d modify this:
and just figure out the URL to start that message (Prefilled composer link generator might help – you just need to know what link you want).