Text-Snippets or shortcuts are a quite handy feature in macOS, especially for writing emails. I often use it to put a greeting at the end of the mail with typing just three letters. Such shortcuts can be defined at

Settings->Keyboard->Text

One problem though is that is not obvious how you can add a line break to the text. To do this you have to press

OPT + RETURN

and viola, you have just added a line break, although it is not really visualized in the Textbox in a nice way…