I had to read the scripts and think about things before I figured this out. There's a slider inside the popover frame. It's been formatted to blend into the background so you can't actually see it.
In layout mode, open the popover and then click the white space just above the portal. You should see selection handles appear around an invisible rectangle slightly larger than the portal. Right click it and select Slider Setup... You can click some arrow buttons to switch back and forth between the select customer and new customer panels of this slider.
Perfect. It seems fairly simple when explained! :) I was fairly certain it was a Slider...I just couldn't seem to access it! You're the man!