@raffzahn already answered with most of what I had in mind when I made that comment. Here are some additional things.
If you have a BASIC program like this
10 INPUT "TYPE SOMETHING";A$
20 PRINT "YOU TYPED ";A$
If you run this, and type something to the prompt, but then type a shift-return instead of a normal return, you will notice that the program does not continue.
Only once you type a normal return it goes on. The text on the line where you typed return will be considered your response.
You would also see an interesting detail in how the INPUT command works in combination with the screen editor. When the INPUT starts, the kernel remembers the cursor's horizontal position. This is so that when you hit return, it can start returning the first character from that screen position, one by one, to the last non-space on the line. The characters are indeed plucked from the screen memory and converted to PETSCII for this!
If you move the cursor off the line, this start position is cleared, because it now makes no sense any more. Sometimes people accidentally type a cursor up or down at an INPUT prompt, and then correct it and go back to the line of the prompt. To their surprise, the prompt is now part of their resulting response. You get the same effect if you use shift-return to go to another line.
Like other control characters, shift-return would have a string representation. For example, Control-Q (cursor down) is (in a string) represented as a reverse-field Q. Let's write that as [Q]. Likewise, you can try to enter shift-return into a program:
10 PRINT "STUFF [shift-M]MORE STUFF"
Except that you can't type it like other cursor control characters: shift-return is sufficiently special that when printing it, even in quote mode or insert mode, it is executed rather than printed as a control character. And using the screen editor involves a small loop that gets characters from the keyboard buffer and prints them according to the normal rules of printing.
So how do you type this line? It just takes two steps. First you type the line and leave a placeholder character instead of the [shift-M]. After you type the 2nd quote you're out of quote mode (or you can type return to get out of quote mode), and you can move the cursor back to the placeholder. Type RVS-ON, and then shift-M. This sort of fabricates the character in another way. Then you type return (again).
Now if you LIST this, you get to see how the [shift-M] gets executed, even between quotes, and not printed as [shift-M].
10 PRINT "STUFF
This is actually fairly close to what you were trying to achieve in your question. There is just not an easy way to edit this line; you'll have to re-create it to change it.
You can try the same with a normal return: [M] instead of [shift-M].
The [M] works so well like when typing it, you only get the first part of the line. Basic's line input loop thinks it is done when it receives character number 13, the [M].