If you wish to send a message to your business partner over ecratum, you have two options :
Leaving a comment on a task
Tasks are how you request information or action from your business partners. You can read more about them here : How Can I Work With Tasks?
Each of the tasks you send or receive has a dedicated page which contains all the information about that task.
Notably you can leave a comment about the task, and decide to make that comment public or only visible to your colleagues.
Read more about how to comment on tasks here : How Can I Reply To Tasks?
Use ecratum messenger
The ecratum messenger let users to send a quick message to your contacts over ecratum. It is meant for quick 'instant message' communication.
To get further information about how you use the ecratum Messenger, click here.