The sketch you posted uses Serial.read() to get characters from the Serial Monitor, but the Serial Monitor will not send any characters until you press Return unless there is a trick that I am not aware of.
Unless there is such a trick then the sketch does not meet these requirements
There is. Press return. Otherwise use a pushbutton, like I did, to call the same function. I don't think the project in question is rocket surgery. Alternatively, change the 'x' to '\n'
@pinknami says that he/she want the Arduino to do something
not when a key followed by Return is pressed
Because of the requirement to react to one of many keys just pressing Return is not a solution, which is where I was leading when I asked
He/she did not reply when I asked
but taking the requirement literally (no use of Enter/Return) then it is not possible using the Serial Monitor but trivial using Coolterm
I still have my doubts that a PC needs to be involved at all but @pinknami has gone quiet
SUMMARY
We don't really know what @pinknami wants or needs and I suspect that neither does he/she
You've got that right: another satisfied customer 
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