Well it has been about 15 years ago so memory is not so great.
Pretty sure it was a WABCO ABS module and yours looks like Bendix so not sure if any of this is applicable. But I think if the light flashes when you start, it has it enabled.
I was reading the manuals and discovered that if it sees an ABS apply valve on the computer output, it is enabled simple as that. I also had to wire the light in the dash and a push button to disable so if you really want to spin the tires, you still can. (It will send a CAN command to cut throttle if both sides are spinning.)
I had to scrounge up two Deutsch pins to populate the empty spots in the connector for the valve, plumb the valve (treadle in - tank air in - delivery to relay valve), and wire the dash.
I remember doing something to verify it had control of the throttle (idle would drop about 100 RPM for a second).
Then I took it to a steep driveway, dumped a pile of ice in front of one tire, drove up on it, stopped and then mashed the throttle (automatic). The tire spun about half a rev, then you heard some air noises and the truck climbed right off the pile of ice.
You could do the ice test yourself if you think it already has it.