Please find out from the manual or your equipment supplier whether Register #1 is the MODBUS holding register #1(in Modicon this is usually labelled as 40002). If so, then you can just use:
WRITEMODBUS 13, 1, 1, &H8
13 - First parameter means to use RTU to write to COMM3.
1 - Assume slave ID = 01
1 - Register #1, which correspond to 40002
&H8 - hexadecimal representation of the number 0008H