This shows what pins are needed for the Yaesu FT-857D to work FT-8 and keying an amp or antenna switch relay simultaneously. However I still recommend connecting all the wires on the pigtailed 8 pin Mini Din connectors. That will make the splitter box useful for other applications.
This is from the FT-857D manual.
It is important to know that pins TX GND and GND are active whether the radio’s settings are set for CAT Control or Linear.