You can choose on the web whether the Enter key should send a message or whether it should create a new line in the message that you are typing.
Once you select the option Send messages with Enter, each time you press Enter, the message will be sent. To add a new line in a message, you will have to use the key combination Shift+Enter.
If you select the option Send messages with Ctrl+Enter, you will need to push both keys to send the message. Clicking on Enter will only add a new line in the existing message.
This setting can be found in the added "General Settings" section (only in your browser):
Navigate to your profile picture on the top right
In the General Settings, set the option you prefer to be activated.