If you use the PLC as slave, then no need for any .PC6 files other than to set the baud rate to that of the same of the touch screen (or if you set the touch screen to that of the PLC, then nothing in the PLC's program need to specifically work with the touch screen). Of course you may choose to use the touch screen to change some variables (e.g. some DMs) and in your .PC6 program you can then access these variables for your program use.
If you use the PLC as master, then you need to use READMODBUS, WRITEMODBUS, READMB2 or WRITEMB2 commands to interact with the touch screen. TBASIC can easily handle that too. So the choice is yours to make.