Register description PCF8574
To find information about registers (addresses, contents) read the datasheet. This applies to every IC.

PCF8754 has got only one register (see p.7), which isn't a typical architecture for an I2C I/O expander1.  The details on how to read and write to it are in sections 8.2 and 8.3 in the datasheet.

1 For a more typical register arrangement, see PCA9555.  The PCF8754 is an older part.  The PCA9555 is a newer part.

Since there is only one register, the very first byte which the bus master sends over the I2C is data: the contents of the register. The register address isn't sent.  Your software library function, however, semantically separates the register address byte from data bytes.   


Look for another library function that doesn't have a parameter for register address.  
Or, write your own such function.  
Or, use the byte address for sending data.  (That would be a hack, though. Somebody else reading your code later might not appreciate it.)