If the event you're participating has enabled the ability, coaches and managers have the ability to chat with the opposing coaches or managers. This is typically used to coordinate a date/time/field of a match to be played.
From your coach/manager profile, click on the "Team Management" button:
Click on the "Matches" button and then filter by your team/event and then click "Search"
For the match that you'd like to contact the opponent for, click on the corresponding chat bubble button:
Here you would be able to view any existing match chat data, or you can click on "New Message" to send one: