In Band-in-a-Box, head to:

Melody | Melodist...

Within this window, you have many options to choose from and select.

To generate JUST the chords, simply remove the check-marks next to the other criteria:
-Generate Intro
-Insert Bass Pedals
-Generate Melody
-Solo in Middle Chorus (may already be unchecked, mine is!)
-Allow Style Change
-Auto-Titles

Make sure there's just a check-mark next to "Generate Chords" and then click [OK]

Voila a complete Chord Progression!

Give it a try on a new project to familiarize yourself with the Melodist feature.

We also have some details on The Melodist within our Online Manual, if you're interested in reading about it:
https://www.pgmusic.com/manuals/bbw2020full/chapter6.htm#automatic-songs-the-melodist

-Callie