Hi Pieter,
Use an additional backslash character (\) to escape each backslash character. I have tested this with PCo 2.305.2141.1051 and it works correctly:
CALL SEND(address='127.0.0.1', port='20000', message='\\16\\16\\05 0115AF\\04');
Just curious, which PLC model are you connecting to?
Regards, Steve