user19933: I know I can send a newline using the option in the bottom right:

But why doesnt it work to use the option 'No line ending', and send a '\n' or '\r\n' after the text using the arduino serial monitor?

user612: The backslash escape encoding for special characters is a feature of double quotes in some programming languages, shells, and other parsers.  It is not normally a feature of live text input - an "end user" human typing a backslash is assumed to mean it literally.
"Live" input of a carriage return or newline is more traditionally done with the return/enter key, or with the corresponding control codes - CTRL-J for newline and/or CTRL-M for return.  I don't know that the Arduino IDE's Serial Monitor supports either of these, at least once you select "no line ending"

19933: <p>I know I can send a newline using the option in the bottom right:</p>

<p><a href="https://i.stack.imgur.com/Kde31.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/Kde31.png" alt="enter image description here"></a></p>

<p>But why doesnt it work to use the option 'No line ending', and send a '\n' or '\r\n' after the text using the arduino serial monitor?</p>
 612: <p>The backslash escape encoding for special characters is a feature of double quotes in some programming languages, shells, and other parsers.  It is not normally a feature of live text input - an "end user" human typing a backslash is assumed to mean it literally.</p>

<p>"Live" input of a carriage return or newline is more traditionally done with the return/enter key, or with the corresponding control codes - CTRL-J for newline and/or CTRL-M for return.  I don't know that the Arduino IDE's Serial Monitor supports either of these, at least once you select "no line ending"</p>
