Add a custom button in the "MessageComposer" or "EmailComposer"

I need to add a custom button that, when clicked, will perform the same function as sending an email, but with an additional function that executes along with the sending. However, I'm unable to access the "MessageComposer" or "EmailComposer" component to implement it. Could you help me with this problem?

In the image below, this is how I want the button (outlined in red) to look when implemented.