How to add "Rules" button on composer like this?

[Febian]

Hello,

I want to add "Rules" button on composer like this. When user click the button, it should open a popup which contain our forum rules (like what "Help" button does). Anyone know how to do it?

Thank you so much for your help :)

[xuso]

I think using the client hook action:composer.loaded and adding the button via Javascript is the best way.

[PitaJ]

The popup is a bootstrap modal.