option key (or the Alt key on a standard keyboard) while you press the Enter/Return

that will add the blank 3️⃣ line to separate the text in a single texT message. Add Line

iMessage on Mac - PosiTek positek : 3️⃣ add-line-

new line/paragraph - Alvin Alexander

inalexander : misc macos-messages-app-how-add-neW-line-parag...