Debug Peoplesoft IB REST 3rd party POST error ( 400 Syntax )?

I am new to API/REST messaging and not sure where to start debugging. I am a veteran of tracing / logging Application Stuff, and even BIP to a degree. But what I don't know is what logs to check for more details on errors in Integration Broker when a REST POST message gets a 400 return code for bad syntax. We are PT8.58.17, it is a simple POST with credentials in the body, to get back tokens.

So... what are the log files to look in? What do I need to change configuration-wise to get more detail?

I figure if I can get output from Peoplesoft with the way I have it configured, then I can compare that to what works in Postman.