Agent Workspace Chat messaging issue

Hi, I have a question about the agent workspaces. I have used the following endpoint and body in order to submit a message in the ticket:

After sending this request I typed a message in the message box (in Zendesk's agent workspace) and clicked send. As you can see from the screenshot below, only the message sent in Zendesk's interface got returned to the visitor's chat window (even though the postman request's message is visible agent workspace).

Would anyone have a solution to this issue?