Not sure if the SKR pro v1.2 actually has Tx & Rx (serial) ports as such. Certainly no spares on the board labelled Tx and Rx.
There are serial ports, but they are used for the TFT/display. There is a USB (B) port that I think is effectively a serial connection.
If you plan on using the pendant in place of the display, you may be able to use one of the TFT or Display1/Display2 ports. You should be able to determine which pins are which by using Google to search for the board pinouts…
Alternatively you may be able to find a Serial-USB converter and wire into the DB9 pins from your pendant.
Best of luck