I've found Shift+Enter to do this pretty reliably across systems whatever they've chosen Enter / Ctrl+Enter to do.

It even works inside bullet points to add separate lines as part of the same bullet.

This is my thinking. Ctrl-Enter is usually "submit the form this input is a part of" in my experience, especially if you're in a multilinear text input (or textarea).

I've seen Enter, Shift-Enter, Ctrl-Enter, and Alt-Enter, (and on macOS, Cmd-Enter and Option-Enter), depending on the application. Total circus. I think this is actually a weakness of the standard keyboard: Keyboards should at the very least separate "submit form / enter" from "newline / carriage return" with different physical keys, but good luck changing that one, given the strong legacy of combining these functions.