How to go to the next line in slack without sending?

The keyboard shortcut to keep entering messages(or paragraphs) to the next line is:

Slack message editing to the next line, without sending.

Alt + Enter

By following this, your message(s) won’t be sent, you can continue editing. The same way multiple lines or paragraphs can be entered, as long as you keep pressing Alt + Enter when you require a new line.

Tip: Another way to do this, is to copy the entire message (with multiple lines, paragraphs, etc) from any other text editor and paste it in the chat window.