I2C control of pcf8574 (no replies)

I am using a BTT SKR 1.4 Turbo board which has limited additional I/O pins and i would like to use an I2C expander based on the pcf8574 chip to control basic on / off functions.

I have the board working with wires.h with a arduino nano without any issues.

Can anyone help with pointers to where and how in the Marlin firmware i can define the addtional 8 pins and allow Marlin to control them, so i can use them in a pins file.

thanks