If you are using the App Studio GUI, you could take a look at the App Studio User's Guide. Look for the section titled 'Creating Drill-Down Hyperlinks Using the Drill Down Command'. If you are hand coding, another method besides Erin's is to use style sheet syntax:
TYPE=type, [subtype], URL=url[(parameters ...)], [TARGET=frame,] [ALT = 'description',] $
You can refer to a report column in the URL component if the URL is dynamic based on report data. You can find more information on this in the Creating Reports with WebFOCUS Language manual.