Trigger: On Scroll
If someone scrolls on your website they must be at least a little bit interested in what you offer. Keep their interest up by offering them something relevant. With Hollerbox you can say how much past the page must done go before the popup comes up.
When editing your popup, you will want to go into Triggers.
Inside of Edit Triggers, turn on On Scroll, then adjust how much you want the person to scroll before the popup comes up.
When you are happy with it, you can close Edit Trigger (top-right corner), then save / publish the popup.