How to add custom functionality to message composer send button?

Hello,
 

At the moment, the Message Composer element is not customizable in the system. We recognize that this limitation may affect your workflow and the flexibility you need to tailor the system to your specific business requirements.
 

That being said, we want to assure you that we are actively working toward enhancing this functionality. Based on the feedback we've received from you and other users, we've increased the priority of this task.

Your experience and satisfaction with the product are very important to us. We constantly strive to make Creatio more adaptable and user-friendly, and your feedback plays a key role in guiding these improvements

Thank you for your patience and understanding.