sylarhero:
I would like to put some data manually to the tx pin and send it immadiately to the rx anddicho process the received data.
I don't understand that.
You say you have a Leonardo. On the Leonardo the pins 0 and 1 are for Serial1. And Serial is used for communication with the PC over the USB cable.
Do you mean that you want the Leonardo to receive data using Serial1 and then send the data to the Arduino Serial Monitor using Serial? That is perfectly possible.
But if you are doing that you need to tell us what you are connecting to Serial1 (pins 0 and 1) to send data to the Leonardo.
...R
Serial Input Basics