user63601: Is there a way to define a different baud rate from the ones we are shown in the dropdown menu of the Serial Monitor? I want to define a custom value.

user7508: There is no easy way to do so. The list of available baud rates is
hard-coded in the Java source of the Arduino IDE. To add a custom
rate, you would have to modify the source code and rebuild the
IDE.
I suggest you consider using a terminal emulator instead of the
serial monitor. On Linux, I like picocom: not a real terminal
emulator, but a minimal program that “connects” the terminal emulator
you are already using (gnome-terminal, rxvt...) to the serial port. It
supports custom baud rates, which it rounds to the nearest
rate the hardware can provide. Picocom works on Mac too.
For Windows, Adafruit recommends Putty or X-CTU. Sparkfun
recommends quite a few options for Windows, Mac and Linux.

63601: <p>Is there a way to define a different baud rate from the ones we are shown in the dropdown menu of the Serial Monitor? I want to define a custom value.</p>
 7508: <p>There is no easy way to do so. The list of available baud rates <a href="https://github.com/arduino/Arduino/blob/1.8.19/app/src/processing/app/AbstractMonitor.java#L24" rel="nofollow noreferrer">is
hard-coded in the Java source</a> of the Arduino IDE. To add a custom
rate, you would have to modify the source code and <a href="https://github.com/arduino/Arduino/wiki/Building-Arduino" rel="nofollow noreferrer">rebuild the
IDE</a>.</p>
<p>I suggest you consider using a <a href="https://en.wikipedia.org/wiki/Terminal_emulator" rel="nofollow noreferrer">terminal emulator</a> instead of the
serial monitor. On Linux, I like <a href="https://github.com/npat-efault/picocom" rel="nofollow noreferrer">picocom</a>: not a real terminal
emulator, but a minimal program that “connects” the terminal emulator
you are already using (gnome-terminal, rxvt...) to the serial port. It
supports <a href="https://github.com/npat-efault/picocom#some-notes-on-custom-baudrate-support" rel="nofollow noreferrer">custom baud rates</a>, which it rounds to the nearest
rate the hardware can provide. Picocom works on Mac too.</p>
<p>For Windows, <a href="https://learn.adafruit.com/windows-tools-for-the-electrical-engineer/serial-terminal" rel="nofollow noreferrer">Adafruit recommends</a> Putty or X-CTU. <a href="https://learn.sparkfun.com/tutorials/terminal-basics/all" rel="nofollow noreferrer">Sparkfun
recommends</a> quite a few options for Windows, Mac and Linux.</p>
